Adaptive beam forming with multi-user detection and interference reduction in satellite communication systems and methods

ABSTRACT

Satellite communications methods include receiving communications signals including co-channel interference at a space-based component from a plurality of wireless terminals in a satellite footprint over a satellite frequency band and reducing interference in the communication signals by (a) performing co-channel interference reduction on the communications signals to generate a plurality of interference reduced signals and (b) performing multiple access interference cancellation on the interference reduced signals. An interference reducing detector for a satellite communications system includes an interference reducer configured to perform co-channel interference reduction on communications signals to generate a plurality of interference reduced signals, and a detector configured to perform multiple access interference cancellation on the interference reduced signals. Satellite communications systems and satellite gateways including interference reducing detectors are also disclosed.

CLAIM OF PRIORITY

This application is a divisional application of U.S. application Ser.No. 11/324,711 filed Jan. 3, 2006, which claims the benefit ofprovisional Application No. 60/641,560, filed Jan. 5, 2005, entitled“Adaptive Beam-Forming with Interference Suppression and Multi-UserDetection in Satellite Systems with Terrestrial Reuse of Frequencies,”the disclosure of which is hereby incorporated by reference herein as ifset forth in its entirety.

FIELD OF THE INVENTION

The present invention relates to interference reduction incommunications systems. In particular, the present invention relates tointerference reduction in satellite communications systems and methodswith terrestrial frequency use/re-use of satellite band frequencies.

BACKGROUND

Satellite communications systems and methods are widely used forradiotelephone communications. Satellite communications systems andmethods generally employ at least one space-based component, such as oneor more satellites, that is/are configured to wirelessly communicatewith a plurality of wireless terminals.

A satellite communications system or method may utilize a single antennabeam or antenna pattern covering an entire area served by the system.Alternatively, or in combination with the above, in cellular satellitecommunications systems and methods, multiple beams (cells or antennapatterns) are provided, each of which can serve a substantially distinctgeographic area in an overall service region, to collectively serve anoverall satellite footprint. Thus, a cellular architecture similar tothat used in conventional terrestrial cellular radiotelephone systemsand methods can be implemented in cellular satellite-based systems andmethods. The satellite typically communicates with wireless terminalsover a bidirectional communications pathway, with wireless terminalcommunications signals being communicated from the satellite to awireless terminal over a downlink or forward link (also referred to as aforward service link), and from the wireless terminal to the satelliteover an uplink or return link (also referred to as a return servicelink).

The overall design and operation of cellular satellite communicationssystems and methods are well known to those having skill in the art, andneed not be described further herein. Moreover, as used herein, the term“wireless terminal” includes devices which include a radio frequencytransceiver, such as cellular and/or satellite radiotelephones; PersonalCommunications System (PCS) terminals that may combine a radiotelephonewith data processing, facsimile and/or data communications capabilities;Personal Digital Assistants (PDA) that can include a radio frequencytransceiver and/or a pager, Internet/Intranet access, Web browser,organizer, calendar and/or a global positioning system (GPS) receiver;and/or conventional laptop and/or palmtop computers or other appliances,which include a radio frequency transceiver. As used herein, the term“wireless terminal” also includes any other radiating userdevice/equipment/source that may have time-varying or fixed geographiccoordinates, and may be portable, transportable, installed in a vehicle(aeronautical, maritime, or land-based), or situated and/or configuredto operate locally and/or in a distributed fashion over one or moreterrestrial and/or extraterrestrial locations. A wireless terminal alsomay be referred to herein as a “radiotelephone,” “radioterminal,”“mobile terminal,” “mobile user terminal,” “user device” or simply as a“terminal”. Furthermore, as used herein, the term “space-based”component includes one or more satellites and/or one or more otherobjects/platforms (e.g., airplanes, balloons, unmanned vehicles, spacecrafts, missiles, etc.) that have a trajectory above the earth at anyaltitude. In addition, as used herein the term “canceling” or“cancellation” as relating to interference canceling or cancellationmeans complete elimination of at least one component/element of theinterference and/or at least a reduction of at least onecomponent/element of the interference.

A terrestrial network that is configured to provide wirelesscommunications by using and/or reusing at least some of the frequenciesauthorized for use by a satellite system can enhance the availability,efficiency and/or economic viability of the satellite system.Specifically, it is known that it may be difficult for satellitecommunications systems to reliably serve densely populated areas,because satellite signals may be blocked by high-rise structures and/ormay not effectively penetrate into buildings. As a result, the satellitespectrum may be underutilized or unutilized in such areas. Theterrestrial use and/or reuse of at least some of the satellite systemfrequencies can reduce or eliminate this potential problem.

Moreover, a capacity measure of an overall system, including aterrestrially-based and a space-based network, may be increased by theintroduction of terrestrial frequency use/reuse of at least some of thefrequencies authorized for use by the space-based network, sinceterrestrial frequency use/reuse may be much denser than that of asatellite-only (space-based network only) system. In fact, capacity maybe enhanced where it may be most needed, i.e., in densely populatedurban/industrial/commercial areas. As a result, the overall system maybecome more economically viable, as it may be able to serve a largersubscriber base more effectively and reliably.

One example of terrestrial reuse of satellite frequencies is describedin U.S. Pat. No. 5,937,332 to Karabinis entitled SatelliteTelecommunications Repeaters and Retransmission Methods, the disclosureof which is hereby incorporated herein by reference in its entirety asif set forth fully herein. As described therein, satellitetelecommunications repeaters are provided which receive, amplify, andlocally retransmit the downlink/uplink signal received from asatellite/radioterminal thereby increasing the effective downlink/uplinkmargin in the vicinity of the satellite telecommunications repeaters andallowing an increase in the penetration of uplink and downlink signalsinto buildings, foliage, transportation vehicles, and other objectswhich can reduce link margin. Both portable and non-portable repeatersare provided. See the abstract of U.S. Pat. No. 5,937,332.

Radioterminals for a satellite communications system or method having aterrestrial communications capability by terrestrially using and/orreusing at least some of the frequencies of a satellite frequency bandthat is also used, at least in part, by the radioterminals forspace-based communications, wherein the radioterminals are configured tocommunicate terrestrially and via a space-based component by usingsubstantially the same air interface for both terrestrial andspace-based communications, may be more cost effective and/oraesthetically appealing than other alternatives. Conventional dualband/dual mode wireless terminal alternatives, such as the well knownThuraya, Iridium and/or Globalstar dual mode satellite/terrestrialwireless terminals, duplicate some components (as a result of thedifferent frequency bands and/or air interface protocols betweensatellite and terrestrial communications), which may lead to increasedcost, size and/or weight of the wireless terminal. See U.S. Pat. No.6,052,560 to Karabinis, entitled Satellite System Utilizing a Pluralityof Air Interface Standards and Method Employing Same.

Satellite communications systems and methods that may employ terrestrialreuse of satellite frequencies are described in U.S. Pat. No. 6,684,057to Karabinis, entitled Systems and Methods for Terrestrial Reuse ofCellular Satellite Frequency Spectrum; and Published U.S. PatentApplication Nos. US 2003/0054760 to Karabinis, entitled Systems andMethods for Terrestrial Reuse of Cellular Satellite Frequency Spectrum;US 2003/0054761 to Karabinis, entitled Spatial Guardbands forTerrestrial Reuse of Satellite Frequencies; US 2003/0054814 to Karabiniset al., entitled Systems and Methods for Monitoring Terrestrially ReusedSatellite Frequencies to Reduce Potential Interference; US 2003/0073436to Karabinis et al., entitled Additional Systems and Methods forMonitoring Terrestrially Reused Satellite Frequencies to ReducePotential Interference; US 2003/0054762 to Karabinis, entitledMulti-Band/Multi-Mode Satellite Radiotelephone Communications Systemsand Methods; US 2003/0153267 to Karabinis, entitled WirelessCommunications Systems and Methods Using Satellite-Linked RemoteTerminal Interface Subsystems; US 2003/0224785 to Karabinis, entitledSystems and Methods for Reducing Satellite Feeder LinkBandwidth/Carriers In Cellular Satellite Systems; US 2002/0041575 toKarabinis et al., entitled Coordinated Satellite-Terrestrial FrequencyReuse; US 2002/0090942 to Karabinis et al., entitled Integrated orAutonomous System and Method of Satellite-Terrestrial Frequency ReuseUsing Signal Attenuation and/or Blockage, Dynamic Assignment ofFrequencies and/or Hysteresis; US 2003/0068978 to Karabinis et al.,entitled Space-Based Network Architectures for Satellite RadiotelephoneSystems; US 2003/0143949 to Karabinis, entitled Filters for CombinedRadiotelephone/GPS Terminals; US 2003/0153308 to Karabinis, entitledStaggered Sectorization for Terrestrial Reuse of Satellite Frequencies;and US 2003/0054815 to Karabinis, entitled Methods and Systems forModifying Satellite Antenna Cell Patterns In Response to TerrestrialReuse of Satellite Frequencies, all of which are assigned to theassignee of the present invention, the disclosures of all of which arehereby incorporated herein by reference in their entirety as if setforth fully herein.

Some satellite communications systems and methods may employinterference cancellation techniques to allow increased terrestrialuse/reuse of satellite frequencies. For example, as described in U.S.Pat. No. 6,684,057 to Karabinis, cited above, a satellite communicationsfrequency can be reused terrestrially by an ancillary terrestrialnetwork even within the same satellite cell that is using the satellitecommunications frequency for space-based communications, usinginterference cancellation techniques. Moreover, the ancillaryterrestrial network can use a modified range of satellite band forwardlink frequencies for transmission, to reduce interference with at leastsome out-of-band receivers. A modified range of satellite band forwardlink frequencies that is used by the ancillary terrestrial network caninclude only a subset of the satellite band forward link frequencies toprovide a guard band between frequencies used by the ancillaryterrestrial network and frequencies used by out-of-band receivers, caninclude power levels that monotonically decrease as a function ofincreasing/decreasing frequency and/or can include two or morecontiguous slots per frame that are left unoccupied and/or aretransmitted at reduced maximum power. Time division duplex operation ofthe ancillary terrestrial network may also be provided over at least aportion of the satellite band return link frequencies. Full or partialreverse mode operation of the ancillary terrestrial network also may beprovided, where at least some of the forward link and return linkfrequencies are interchanged with the conventional satellite forwardlink and return link frequencies. See the Abstract of U.S. Pat. No.6,684,057.

SUMMARY

Satellite communications methods according to embodiments of theinvention include receiving at a space-based component a plurality ofmultiple access signals from a plurality of terminals in a footprint ofthe space-based component over a frequency band of the space-basedcomponent, the plurality of multiple access signals includinginterference that is dependent on signals transmitted by the terminalsand interference that is independent of signals transmitted by theterminals; and reducing interference of the plurality of multiple accesssignals by first reducing the interference that is independent of thesignals transmitted by the terminals followed by canceling theinterference that is dependent on the signals transmitted by theterminals.

Some methods further include receiving/transmitting wirelesscommunications signals at an ancillary terrestrial component from/to aplurality of terminals in the satellite footprint over the satellitefrequency band. The space-based component may also receive the wirelesscommunications signals as interference to the multiple access signals.

Receiving multiple access signals at a space-based component from aplurality of terminals in a satellite footprint over a satellitefrequency band may include receiving multiple access signals using anantenna including a plurality of antenna feed elements that may beconfigured to provide antenna patterns that differ in spatialorientations therebetween and wherein at least some of the antenna feedelements may also be configured to receive electro-magnetic energy overat least two different polarization orientations.

Reducing interference that is independent of the signals transmitted bythe terminals may include performing co-channel interference reductionon a multiple access signal, including a pilot signal and an informationsignal, transmitted by a terminal and received by a plurality of antennafeed elements. Such interference reduction may include processing of thepilot signal and determining a set of weights for the plurality ofantenna feed elements based on the processing of the pilot signal.

Methods according to some embodiments of the invention may furtherinclude generating at least one pilot signal error based on theprocessing of the pilot signal.

The set of weights for the plurality of antenna feed elements may beselected to reduce a mean squared measure of the pilot signal errorthereby providing an interference reduced received pilot signal, andsome methods according to the invention further include applying the setof weights to signals received by the plurality of antenna feed elementsto obtain an interference reduced received information signal.

Performing multiple access interference cancellation (or at leastinterference reduction) on the interference reduced received informationsignal may include determining a set of channel estimates based oninterference reduced received information signals and/or interferencereduced received pilot signals, generating a set of received informationestimates (e.g., bit estimates) from the interference reduced receivedinformation signals, and performing multiple access interferencecancellation (or at least interference reduction) on the interferencereduced received information signals using the set of channel estimatesand the information estimates.

Performing multiple access interference cancellation on the interferencereduced received information signals using the channel estimates and theinformation estimates may include generating second interference reducedreceived information signals and/or second interference reduced receivedpilot signals. Moreover, methods according to embodiments of theinvention may further include determining a set of second channelestimates based on the second interference reduced received informationsignals and/or second interference reduced received pilot signals,generating a set of second received information estimates from thesecond interference reduced received information signals, and performingmultiple access interference cancellation on the second interferencereduced received information signals using the second channel estimatesand the second information estimates.

Some methods further include receiving at the space-based componentusing at least two antenna patterns that differ in at least apolarization orientation.

Generating a set of received information estimates from the interferencereduced received information signals may include correlating theinterference reduced received information signals with a set of knownsignal spreading codes used by the plurality of terminals.

Performing multiple access interference cancellation on the interferencereduced received information signals may include generating a pluralityof interference reduced information estimates, and some methods mayfurther include performing multiple access interference cancellationusing the plurality of interference reduced information estimates.

Methods according to further embodiments of the invention may furtherinclude re-transmitting the multiple access signals to a satellitegateway, and reducing interference in the multiple access signals may beperformed at the satellite gateway which may be terrestrially-based.Further, performing multiple access interference cancellation on theinterference reduced received information signals may be performed atthe satellite gateway.

Some methods may further include repeatedly reducing the interferencethat may be dependent on the transmissions of the plurality of terminalscommunicating with the space-based component over the geographic areauntil a predetermined criterion may be met. The predetermined criterionmay include a bit error rate.

A cellular satellite system according to further embodiments of theinvention includes a space-based component and a plurality of terminalsthat are configured to transmit a respective plurality of multipleaccess signals, comprising pilot signals and information signals, over asatellite frequency band in a satellite footprint; the space-basedcomponent configured to receive the plurality of multiple access signalsover the satellite frequency band, the space-based component alsoreceiving interference along with the plurality of multiple accesssignals in the satellite frequency band, and an interference reducerthat is responsive to the space-based component, and that is configuredto sequentially perform co-channel interference reduction and multipleaccess interference cancellation on the plurality of multiple accesssignals.

Some systems may further include an ancillary terrestrial networkincluding a plurality of terminals wherein the ancillary terrestrialnetwork and/or the terminals are/is configured to transmit wirelesscommunications signals over the satellite frequency band in thesatellite footprint.

The space-based component may include an antenna having a plurality ofantenna feed elements, and the space-based component may be configuredto receive the plurality of multiple access signals using the antenna.

The interference reducer may be further configured to perform co-channelinterference reduction on the multiple access signals received from theplurality of terminals, by processing pilot signals transmitted by theplurality of terminals and received by the space-based component anddetermining a set of weights for the antenna feed elements based on theprocessing of the pilot signals.

The interference reducer may be further configured to generate at leastone pilot signal error based on the processing of the pilot signals.

The interference reducer may be further configured to determine a set ofweights for the antenna feed elements to reduce a mean squared measureof the at least one pilot signal error thereby providing an interferencereduced pilot signal.

The interference reducer may be further configured to apply the set ofweights to signals received by the plurality of antenna feed elements toobtain an interference reduced received information signal.

The interference reducer may be further configured to determine a set ofchannel estimates based on interference reduced received informationsignals and/or interference reduced pilot signals, generate a set ofreceived information estimates (e.g., bit estimates) from theinterference reduced received information signals, and perform multipleaccess interference cancellation on the interference reduced receivedinformation signals using the set of channel estimates and theinformation estimates to thereby generate second interference reducedreceived information signals.

The interference reducer may be further configured to determine a set ofsecond channel estimates based on the second interference reducedreceived information signals, to generate a set of second received bitestimates from the second interference reduced received informationsignals, and to perform multiple access interference cancellation on thesecond interference reduced received information signals using thesecond channel estimates and the second bit estimates.

The space-based component may be configured to receive multiple accesssignals using at least two antenna patterns that differ in spatialorientation therebetween and/or wherein at least two antenna patternsdiffer in a polarization orientation.

The interference reducer may be further configured to generate aplurality of interference reduced bit estimates from the interferencereduced received information signals, and to perform multiple accessinterference cancellation using the plurality of interference reducedbit estimates.

The space-based component may be further configured to re-transmit themultiple access signals to a satellite gateway, and the interferencereducer may be located at the satellite gateway, which may beterrestrially-based.

A satellite wireless terminal system according to further embodiments ofthe invention includes a space-based component configured to receivemultiple access wireless communications signals from a plurality ofwireless terminals in a satellite footprint over a satellite frequencyband, an interference reducer responsive to the space-based componentand configured to perform co-channel interference reduction on themultiple access wireless communications signals to thereby generate aplurality of interference reduced received information signals, and adetector responsive to the interference reducer and configured toperform multiple access interference cancellation on the interferencereduced received information signals.

Systems according to some embodiments of the invention may furtherinclude an ancillary terrestrial network including a plurality oftransmitters configured to transmit a plurality of wirelesscommunications signals over the satellite frequency band in thesatellite footprint, the space-based component also receiving thewireless communications signals as interference along with the multipleaccess wireless communications signals.

The space-based component may include an antenna having a plurality ofantenna feed elements, and the space-based component may be configuredto receive the plurality of multiple access wireless communicationssignals using the antenna.

The interference reducer may be further configured to perform co-channelinterference reduction on a multiple access wireless communicationssignal by processing at least one pilot signal transmitted by a wirelessterminal and determining a set of weights for a respective set ofantenna feed elements based on the processing of the at least one pilotsignal.

The interference reducer may be further configured to generate at leastone pilot signal error based on the processing.

The interference reducer may be further configured to select a set ofsignal weights for the antenna feed elements to reduce a mean squaredmeasure of the at least one pilot signal error.

The interference reducer may be further configured to apply the set ofsignal weights to signals received by a plurality of antenna feedelements to obtain an interference reduced received information signal.

The detector may be further configured to determine a set of channelestimates based on interference reduced received information signals,generate a set of received bit estimates from the interference reducedreceived information signals, and perform multiple access interferencecancellation on the interference reduced received information signalsusing the set of channel estimates and the bit estimates to therebygenerate second interference reduced received information signals.

The detector may be further configured to determine a set of secondchannel estimates based on the second interference reduced receivedinformation signals, to generate a set of second received bit estimatesfrom the second interference reduced received information signals, andto perform multiple access interference cancellation on the secondinterference reduced received information signals using the secondchannel estimates and the second bit estimates.

The space-based component may be further configured to receive signalsusing at least two antenna patterns that differ in at least apolarization and/or spatial orientation.

The detector may be further configured to generate a plurality ofinterference reduced bit estimates from the interference reducedreceived information signals, and to perform multiple accessinterference cancellation using the plurality of interference reducedbit estimates.

The space-based component may be further configured to re-transmit themultiple access signals to a satellite gateway, and the interferencereducer may be located at the satellite gateway, which may beterrestrially-based.

Systems according to some embodiments of the invention may furtherinclude a satellite gateway, and the interference reducer may be locatedat the space-based component, the detector may be located at thesatellite gateway, and the space-based component may be furtherconfigured to transmit the interference reduced received informationsignals to the satellite gateway.

Some embodiments of the invention provide an interference reducingdetector for a satellite communications system including a space-basedcomponent configured to receive multiple access wireless communicationssignals including co-channel interference from a plurality of wirelessterminals in a satellite footprint over a satellite frequency band, theinterference reducing detector including an interference reducerresponsive to the space-based component and configured to performco-channel interference reduction on the multiple access wirelesscommunication signals to generate a plurality of interference reducedreceived information signals, and a detector configured to performmultiple access interference cancellation on the interference reducedreceived information signals.

The interference reducer of the interference reducing detector may befurther configured to perform co-channel interference reduction on themultiple access wireless communications signals received from theplurality of wireless terminals by processing pilot signals transmittedby the plurality of wireless terminals and determining sets of weightsfor a respective set of antenna feed elements based on the processing ofthe pilot signals.

The interference reducer of the interference reducing detector may befurther configured to generate at least one pilot signal error based onthe processing.

The interference reducer of the interference reducing detector may befurther configured to select a set of signal weights for the antennafeed elements to reduce a mean squared measure of the at least one pilotsignal error.

The interference reducer of the interference reducing detector may befurther configured to apply the set of signal weights to signalsreceived by a plurality of antenna feed elements to obtain a pluralityof interference reduced received information signals.

The detector of the interference reducing detector may be furtherconfigured to determine a set of channel estimates based on theinterference reduced received information signals, generate a set ofreceived bit estimates from the interference reduced receivedinformation signals, and perform multiple access interferencecancellation on the interference reduced received information signalsusing the set of channel estimates and the bit estimates to therebygenerate second interference reduced received information signals.

The detector of the interference reducing detector may be furtherconfigured to determine a set of second channel estimates based on thesecond interference reduced received information signals, generate a setof second received bit estimates from the second interference reducedreceived information signals, and perform multiple access interferencecancellation on the second interference reduced received informationsignals using the second channel estimates and the second bit estimates.

The space-based component may be further configured to receive signalsusing at least two antenna patterns that differ in at least a spatialand/or polarization orientation.

The detector of the interference reducing detector may be furtherconfigured to generate a plurality of interference reduced bit estimatesfrom the interference reduced received information signals, and toperform multiple access interference cancellation using the plurality ofinterference reduced bit estimates.

The space-based component may be further configured to re-transmit themultiple access wireless communication signals to a satellite gateway,and the interference reducer may be located at the satellite gateway,which may be terrestrially-based.

The interference reducer of the interference reducing detector may belocated at the space-based component and the detector may be locatedremotely from the space-based component.

Some embodiments of the invention provide a gateway for a satellitewireless terminal system that may include a space-based component thatis configured to receive multiple access wireless communications signalsfrom a plurality of wireless terminals in a satellite footprint over asatellite frequency band, the gateway including an interference reducerresponsive to the space-based component and configured to performco-channel interference reduction on the multiple access wirelesscommunications signals to generate a plurality of interference reducedreceived information signals, and a detector that is configured toperform multiple access interference cancellation on the interferencereduced received information signals.

The interference reducer of the gateway may be further configured toperform co-channel interference reduction on the multiple accesswireless communications signals by processing pilot signals transmittedby the plurality of wireless terminals and determining sets of weightsfor a set of antenna feed elements based on the processing of the pilotsignals.

The interference reducer of the gateway may be further configured togenerate at least one pilot signal error based on the processing.

The interference reducer may be further configured to select a set ofsignal weights for the antenna feed elements to reduce a mean squaredmeasure of the at least one pilot signal error.

The interference reducer of the gateway may be further configured toapply the sets of signal weights to signals received by a plurality ofantenna feed elements to obtain the plurality of interference reducedreceived information signals.

The detector of the gateway may be further configured to determine a setof channel estimates based on the interference reduced receivedinformation signals, generate a set of received bit estimates from theinterference reduced received information signals, and perform multipleaccess interference cancellation on the interference reduced receivedinformation signals using the set of channel estimates and the bitestimates to thereby generate second interference reduced receivedinformation signals.

The detector of the gateway may be further configured to determine a setof second channel estimates based on the second interference reducedreceived information signals, generate a set of second received bitestimates from the second interference reduced received informationsignals, and perform multiple access interference cancellation on thesecond interference reduced received information signals using thesecond channel estimates and the second bit estimates.

The space-based component may be further configured to receive signalsusing at least two antenna patterns that differ in at least a spatialand/or polarization orientation.

The detector of the gateway may be further configured to generate aplurality of interference reduced bit estimates from the interferencereduced received information signals, and to perform multiple accessinterference cancellation using the plurality of interference reducedbit estimates.

A method of reducing interference according to some embodiments of theinvention includes receiving, at a space-based component, components ofa signal using at least first and second antenna patterns that differ ina spatial orientation and a polarization orientation, providing thecomponents of the signal to an interference reducer, and processing thecomponents of the signal at the interference reducer to reduce a levelof interference of the signal.

A method of communicating between a space-based component and aradioterminal according to some embodiments of the invention includestransmitting a first signal to the radioterminal over a first antennapattern of the space-based component, and transmitting a second signalto the radioterminal over at least a second antenna pattern of thespace-based component, wherein the second signal differs from the firstsignal by at least a time delay value. The first antenna pattern maydiffer from the second antenna pattern by a spatial orientation and/or apolarization orientation.

A method of communicating with a space-based component according to someembodiments of the invention includes receiving at a radioterminal afirst signal over a first antenna pattern of the space-based componentand at least one second signal over at least a second antenna pattern ofthe space-based component, and processing the first signal and the atleast one second signal at the radioterminal to improve at least onecommunications performance measure. The at least one second signal maydiffer from the first signal by at least a time delay value.

According to some embodiments of the invention, a communications methodfor a wireless communications system including a space-based componentand an ancillary terrestrial network includes providing control channeland traffic channel communications from the ancillary terrestrialnetwork to a plurality of first radioterminals using a first set offrequencies authorized for use by the space-based component to providecontrol channel and/or traffic channel communications more than a secondset of frequencies authorized for use by the space-based component toprovide control channel and/or traffic channel communications, andproviding control channel and traffic channel communications from thespace-based component to a plurality of second radioterminals within ageographic area using the second set of frequencies. In someembodiments, the ancillary terrestrial network may not use the secondset of frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate certain embodiment(s) of theinvention. In the drawings:

FIG. 1 is a schematic diagram of a cellular satellite communicationssystem and methods according to embodiments of the invention.

FIGS. 2A-2C are block diagrams of an interference reducer andconstituent components according to embodiments of the invention.

FIGS. 3A-3B are block diagrams of an interference reducer andconstituent components according to embodiments of the invention.

FIG. 4A-4B are block diagrams of interference reducers according toembodiments of the invention.

FIGS. 5-8 are flowcharts illustrating systems and methods for reducinginterference according to embodiments of the invention.

FIG. 9 illustrates satellite spot beams, some of which include ATCinfrastructure configurations.

FIG. 10 illustrates a gain and phase pattern of an antenna feed element.

FIG. 11 is a block diagram of a single-user interference cancellationdetector according to embodiments of the invention.

FIG. 12 is a block diagram of a multi-user interference cancellationdetector according to embodiments of the invention.

FIG. 13 is a map of the continental United States showing aconfiguration of forward link satellite spot beams and locations oftransmitters of an ancillary terrestrial network.

FIG. 14 is a map of the continental United States showing aconfiguration of return link service areas formed by return link feedelements of a space-based component and locations of transmitters of anancillary terrestrial network.

FIGS. 15 and 16 are graphs of bit error rate (BER) versus signal tointerference ratio (SIR) for various receiver configurations accordingto embodiments of the invention.

FIG. 17 is a graph of Delta T/T increase versus SIR for various receiverconfigurations according to embodiments of the invention.

FIG. 18 is a three dimensional graph of gain versus azimuth/elevationfor an antenna pattern formed by an antenna feed element of aspace-based component.

FIG. 19 is a gain contour pattern of the graph of FIG. 18.

FIG. 20 is a three dimensional graph of gain versus azimuth/elevationfor an adaptively-formed antenna pattern using a plurality of antennafeed elements.

FIG. 21 is a gain contour pattern of the graph of FIG. 20.

FIGS. 22-24 are graphs of bit error rate (BER) versus signal tointerference ratio (SIR) for various receiver configurations accordingto embodiments of the invention.

FIG. 25 is a gain contour pattern of an antenna feed element.

FIGS. 26-33 are graphs of BER under various simulation conditionsaccording to embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

It will be understood that although the terms first and second may beused herein to describe various elements, these elements should not belimited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element below could betermed a second element, and similarly, a second element may be termed afirst element without departing from the teachings of the presentinvention. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. The symbol“/” is also used as a shorthand notation for “and/or”.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

As will be appreciated by one of skill in the art, the present inventionmay be embodied as a method, data processing system, and/or computerprogram product. Accordingly, the present invention may take the form ofan entirely hardware embodiment, an entirely software embodiment or anembodiment combining software and hardware aspects all generallyreferred to herein as a “circuit” or “module.” Furthermore, the presentinvention may take the form of a computer program product on a computerusable storage medium having computer usable program code embodied inthe medium. Any suitable computer readable medium may be utilizedincluding hard disks, CD ROMs, optical storage devices, a transmissionmedia such as those supporting the Internet or an intranet, or magneticstorage devices.

The present invention is described below with reference to flowchartillustrations and/or block diagrams of methods, systems and computerprogram products according to embodiments of the invention. It will beunderstood that each block of the flowchart illustrations and/or blockdiagrams, and combinations of blocks in the flowchart illustrationsand/or block diagrams, can be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

These computer program instructions may also be stored in a computerreadable memory that can direct a computer or other programmable dataprocessing apparatus to function in a particular manner, such that theinstructions stored in the computer readable memory produce an articleof manufacture including instruction means which implement thefunction/act specified in the flowchart and/or block diagram block orblocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

Moreover, as used herein, “substantially the same” band(s) means thattwo to more bands being compared substantially overlap, but that theremay be some areas of non-overlap, for example at a band end and/orelsewhere. “Substantially the same” air interface(s) means that two ormore air interfaces being compared are similar but need not beidentical. For example, a first air interface (i.e., a satellite airinterface) may include some differences relative to a second airinterface (i.e., a terrestrial air interface) to, for example, accountfor one or more different characteristics of acommunications/propagation environment and/or to address otherperformance aspects and/or system concerns associated with the firstand/or second air interface.

For example, a different vocoder rate may be used for satellitecommunications compared to the vocoder rate that may be used forterrestrial communications (e.g., for terrestrial communications, audiosignals may be encoded (“vocoded”) at a rate of approximately 9 to 13kbps or higher, whereas for satellite communications a vocoder rate ofapproximately 2 to 4 kbps may be used). Likewise, a different forwarderror correction code, different interleaving depth, and/or differentspread-spectrum codes may also be used, for example, for satellitecommunications compared to a code, interleaving depth, and/or spreadspectrum codes (i.e., Walsh codes, short codes, long codes, and/orfrequency hopping codes) that may be used for terrestrialcommunications.

The terrestrial use/reuse of satellite-band service-link frequencies hasbeen proposed to, and accepted by, the Federal Communications Commission(FCC) and Industry Canada (IC). See, e.g., Report and Order and Noticeof Proposed Rulemaking, FCC 03-15, “Flexibility for Delivery ofCommunications by Mobile Satellite Service Providers in the 2 GHz Band,the L-Band, and the 1.6/2.4 Bands”, IB Docket No. 01-185, Adopted: Jan.29, 2003, Released: Feb. 10, 2003, and Industry Canada, SpectrumManagement and Telecommunications Policy DGTP-006-04 “Spectrum andLicensing Policy to Permit Ancillary Terrestrial Mobile Services as Partof Mobile-Satellite Service Offerings,” May 2004. Also see, e.g.,Memorandum Opinion and Order and Second Order on Reconsideration, FCC05-30, IB Docket No. 01-185; Adopted: Feb. 10, 2005, Released: Feb. 25,2005.

Some embodiments of the invention may perform adaptive signalprocessing, including beam-forming (i.e., antenna pattern shaping),interference suppression, channel estimation and multi-user detection ina Mobile Satellite System (MSS) environment with terrestrial use/reuseof the satellite band frequencies. Beam-forming, based on a MinimumMean-Squared Error (MMSE) performance index, for example, may be used toincrease a signal-to-noise plus interference ratio of MSS links in anenvironment characterized by significant terrestrial reuse of thesatellite service link frequencies. Elements of an ancillary terrestrialnetwork which use/re-use satellite band frequencies are referred toherein as Ancillary Terrestrial Components (ATCs).

Embodiments of the invention can mitigate both ATC-induced andnon-ATC-induced interference (that may be co-frequency/co-channel and/orout-of-channel/band) and Multiple-Access Interference (MAI) in a MobileSatellite System (MSS) environment. In addition, significant performanceimprovements may be obtained by using both space and time processing ofsignals received at the satellite. In some embodiments, a pilot-basedMMSE algorithm may be used to adaptively form a beam (i.e., antennapattern) for a user by processing a set of antenna feed element signals.Following beam-forming (i.e., antenna pattern forming), pilot signalsmay be used to estimate parameters of user channels. A Sequential ATCand MAI Interference Canceller (SAM IC) in accordance with embodimentsof the invention can take advantage of known pilot signal informationand preliminary decisions of received information to sequentiallyperform interference suppression, followed by multi-user detection. Theperformance of the SAMIC algorithm is illustrated by simulation of amulti-beam geo-stationary satellite system containing a wide deploymentof ATC over 50 major markets of the Continental United States (CONUS).

While the term “interference canceller” and related terms such as“interference cancellation” and “interference canceling” are used hereinto describe elements, systems and methods according to embodiments ofthe invention, it will be appreciated that while some interferencereduction techniques may be referred to as “interference cancellation,”some residual interference may remain in a signal even after“interference cancellation.” That is, as with any physical process,complete elimination of interference may be impossible or impractical,even in so-called “optimal” systems.

FIG. 1 is a schematic diagram of cellular satellite communicationssystems and methods according to embodiments of the invention. As shownin FIG. 1, these cellular satellite communications systems and methods100 include a Space-Based Component (SBC) 110, such as a geostationaryor non-geo-stationary orbiting satellite. The space-based component 110may be configured to selectively use geographically a set of frequenciesand to transmit wireless communications signals to a plurality ofwireless terminals, only one of which is illustrated in FIG. 1 (terminal120 a), in a satellite footprint including one or more satellite cells130-130″″, over one or more satellite forward service link (downlink)frequencies f_(p). The space-based component 110 may also be configuredto receive wireless communications from a plurality of wirelessterminals, such as wireless terminal 120 a in the satellite cell 130,over one or more satellite return service link (uplink) frequenciesf_(u).

An ancillary terrestrial network (ATN), comprising at least oneancillary terrestrial component (ATC) 140, which may include an antenna140 a and an electronics system 140 b, is configured to receive wirelesscommunications signals from, for example, at least one wireless terminal120 b over an uplink frequency, denoted f′_(u), within the satellitefrequency band. The frequency f′_(u) may be the same as an uplink ordownlink frequency used for communicating with the space-based component(SBC) 110 in the satellite cell 130 in which the wireless terminal 120 bis located and/or in an adjacent or remotely-located satellite cell 130.Thus, as illustrated in FIG. 1, the wireless terminal 120 a may becommunicating with the space-based component 110 using a frequency inthe satellite frequency band while the wireless terminal 120 b may becommunicating with the ancillary terrestrial component 140, also using afrequency in the satellite frequency band. As shown in FIG. 1, thespace-based component 110 also undesirably receives a component of thewireless communications from the wireless terminal 120 b and/or the ATC140 in the satellite cell 130 as interference. In addition, the spacebased component 110 may receive a component of wireless communicationsfrom a wireless terminal and/or ATC (not shown) located in a differentsatellite cell over a satellite frequency that may be the same as(and/or overlapping with) f_(U) and/or f′_(U).

More specifically, a potential interference path is shown at 150. Inthis potential interference path 150, the signal transmitted by thewireless terminal 120 b and/or the ATC 140 interferes with satellitecommunications. This interference would generally be strongest when thetransmitted signal uses the same carrier frequency as the cell inquestion (e.g., f′_(U)=f_(U)), because, in that case, the same returnlink frequency would be used for space-based component and ancillaryterrestrial component communications and, if used over the samesatellite cell, no substantial spatial discrimination between satellitecells would appear to exist to reduce a level of interference. Even withspatial separation, however, interference may impair the signal from thefirst wireless terminal 120 a.

Still referring to FIG. 1, embodiments of satellite communicationssystems/methods 100 can include at least one satellite gateway 160 thatcan include an antenna 160 a and an electronics system 160 b. Thesatellite gateway 160 may be connected to other networks 162, includingterrestrial and/or other wired and/or wireless communications networkssuch as, for example, a public switched telephone network and/or theInternet. The satellite gateway 160 communicates with the space-basedcomponent 110 over a satellite feeder link 112. The satellite gateway160 may also be configured to communicate with ancillary terrestrialcomponents 140 in the ancillary terrestrial network, generally over aterrestrial link 142.

Still referring to FIG. 1, an Interference Reducing (IR), signalprocessor 170 also may be provided at least partially in the gatewayelectronics system 160 b. In yet other alternatives, the interferencereducing signal processor 170 may be provided at least partially inother components of the cellular satellite system/method 100 instead ofor in addition to the gateway electronics system 160 b. For example, aninterference reducing signal processor 170 may be at least partiallyprovided in the space-based component 110. The interference reducingsignal processor 170 may be responsive to the space-based component 110and to the ancillary terrestrial component 140, and may be configured toreduce interference from the wireless communications that are receivedby the space-based component 110. In particular, the interferencereducing signal processor 170 may be configured to reduce interferencethat is at least partially generated by ATC's such as ATC 140 andwireless terminals such as wireless terminal 120 b communicating withthe ancillary terrestrial network. In addition, the interferencereducing signal processor 170 may also be configured to reduceinterference from other transmitters such as, for example, transmittersoperating outside the MSS and/or the ATN.

Systems and methods disclosed in this application may be advantageouslyutilized in a system employing terrestrial use/reuse of satellite-bandfrequencies. As described above, the Ancillary Terrestrial Network (ATN)uses/reuses the at least some of the satellite-band service linkfrequencies to provide reliable communications in populous areas wheresatellite connectivity is unreliable. As a consequence of theterrestrial use/reuse of the satellite-band frequencies, uplinkco-channel interference to satellite links may be present and may becomeharmful, under certain conditions, where there is insufficientdiscrimination between satellite and terrestrial links. Embodiments ofthe invention may be advantageously employed in a state-of-the-artMobile Satellite System (MSS) operating in conjunction with an ancillaryterrestrial network that is widely deployed over a plurality of marketsover, for example, the Continental United States (CONUS) and/or othergeographic areas. Some embodiments of the invention may be particularlyapplicable to an MSS/ATN system employing a spread-spectrum multipleaccess communications protocol such as, for example, a cdma2000 1XRTTprotocol. Embodiments of the invention, however, may be applied to anycommunications protocol and/or air interface, as will be recognized bythose skilled in the art.

Multiple access interference (MAI) is a type of co-channel interferencethat may diminish the quality of a signal received at a satellite in amultiple access communications environment. In such an environment,multiple transmitters communicate with a single receiver (such as asatellite receiver) using a shared communicationsmedium/carrier/channel. In general, there are at least three basicmultiple access schemes: time division multiple access (TDMA), codedivision multiple access (CDMA) and frequency division multiple access(FDMA). In an FDMA scheme, different transmitters are assigned differentfrequency bands on which to transmit. In a TDMA system, differenttransmitters are assigned different time slots (i.e., time intervals)within a particular frequency band. Thus, in accordance with a TDMAsystem, a transmitter is assigned to a particular frequency band (as inFDMA), but temporally shares the frequency band in order to improve bandutilization. In a general CDMA scheme, multiple transmitters share asingle, relatively wide frequency band, but the transmitters may not belimited to particular time slots. Rather, each transmitter is assigned aunique spreading code (or “chipping” code) that is in some embodimentsorthogonal to the spreading code used by each of the other transmitters.Information transmitted by each transmitter is modulated using thetransmitter's spreading code. Thus, the signal broadcast by a firstco-frequency (co-channel) transmitter may ideally appear as noise whenadded to the signal transmitted by a second co-frequency (co-channel)transmitter. More advanced multiple access systems may combine aspectsof FDMA, TDMA and/or CDMA. In general, a receiver in a multiple accesssystem may be required to estimate a signal transmitted by a transmitterthat is subject to co-channel MAI due to the signals transmitted byother transmitters in the system.

In conventional third generation (3G) CDMA systems, potentialimpediments of signal detection generally are (i) multipath fading, and(ii) MAI caused by co-channel transmissions using codes that are notorthogonal to the signal of the desired user. Rake matched filtering caneffectively combat multipath fading by coherently combining resolvablemultipath replicas of the desired signal. A receiver comprising amulti-element antenna may be configured to combine rake matchedfiltering with space-time processing of signals to reduce MAI.

A multi-user detection system configured to reduce MAI may be contrastedwith a single-user detection technique which detects a desired usersignal without regard to the MAI. In accordance with some embodiments ofthe present invention, a communications receiver may be configured witha first signal processing stage which is operative on a plurality ofreceived signals provided to the communications receiver by a respectiveplurality of antenna patterns of a space-based component wherein, ingeneral, the plurality of antenna patterns differ therebetween inspatial orientation (i.e., project different gain contours over theservice area of the space-based component) and/or may differtherebetween in one or more polarization orientations. In someembodiments, the plurality of antenna patterns are formed by thespace-based component using at least one antenna feed element of thespace-based component. In some embodiments of the invention, at leastone of the plurality of antenna patterns includes at least twopolarization-distinct antenna patterns providing a signal to thecommunications receiver including at least two components, respectivelyassociated with at least two different polarization orientations of theat least one of the plurality of antenna patterns. In some embodiments,the at least two different polarization orientations include asubstantially Right Hand Circular Polarization (RHCP) and asubstantially Left Hand Circular Polarization (LHCP). In otherembodiments of the invention, each one of the plurality of antennapatterns provides a signal including at least two components,respectively associated with at least two different polarizationorientations. The first signal processing stage of the communicationsreceiver may operate on the plurality of received signals to reduce alevel of interference therein, thereby enabling a second stage of thecommunications receiver, following the first, to more effectively reduceMAI and perform Multi-User Detection (MUD).

In some embodiments, the communications receiver is configured at one ormore satellite gateways. In other embodiments, the communicationsreceiver is configured at a space-based component. In still furtherembodiments, the communications receiver may be distributed between thespace-based component and at least one satellite gateway.

In some embodiments of the invention, the first signal processing stageof the communications receiver, which is operative on a plurality ofreceived signals provided to the communications receiver by a respectiveplurality of antenna patterns of a space-based component, may beselectively operative on a predetermined plurality of received signalsprovided to the communications receiver by a respective predeterminedplurality of antenna patterns of the space-based component. Thepredetermined plurality of received signals may, in some embodiments ofthe invention, be a sub-set of an ensemble of signals received by arespective ensemble of antenna patterns of the space-based component,and the selection of the predetermined plurality of received signals(i.e., the selection of the predetermined plurality of antenna patternsthat provide the predetermined plurality of received signals) may beresponsive to a received return link control channel signal. A locationand/or a geographic area associated with the received return linkcontrol channel signal may, in some embodiments of the invention, beused to select the predetermined plurality of antenna patterns thatprovide the predetermined plurality of received signals.

In some embodiments, the return link control channel signal isconfigured to occupy a frequency range that is not used/re-used or isminimally used/re-used by an Ancillary Terrestrial Network (ATN) and/orother network, thereby minimizing or reducing a level of interferenceassociated with the return link control channel signal. Accordingly, thereturn link control channel signal may be received by the space-basedcomponent substantially free, or at a reduced level, of interferencethat may, otherwise, be caused by terrestrial (and/or other) use/reuseof the return link control channel frequencies. The return link controlchannel signal may be received by the space-based component via one ormore space-based component antenna patterns (beams/cells and/or antennapatterns formed by antenna feed elements). Responsive to the one or morespace-based component antenna patterns that receive the return linkcontrol channel signal and/or responsive to respective return linkcontrol channel signal strength and/or signal quality associated withthe one or more space-based component antenna patterns that receive thereturn link control channel signal, a geographic location associatedwith a source (such as, for example, a radioterminal source) associatedwith the return link control channel signal may be determined and usedto select the predetermined plurality of antenna patterns that providethe predetermined plurality of received signals. Accordingly, relativeto a source that emits a return link control channel signal, thespace-based component may be configured to determine a geographiclocation associated with the source and configure a communicationsreceiver to selectively operate on a predetermined plurality of receivedsignals provided to the communications receiver by a respectivepredetermined plurality of antenna patterns of the space-based componentthat, for the determined geographic location associated with the source,are determined to be optimum or near optimum in enabling thecommunications receiver to establish a maximum or near maximum desiredsignal to interference and/or noise performance measure.

It will be understood that the return link control channel signal may bereceived by the space-based component using substantially fixed spotbeams and/or antenna patterns that may be associated with one or moreantenna feed elements (i.e., receive antenna feed elements) of thespace-based component. In some embodiments of the invention, forwardlink control channel signals may also be based on substantially fixedspot beams and/or antenna patterns that may be associated with one ormore antenna feed elements (i.e., transmit antenna feed elements) of thespace-based component. A forward link control channel signal may beradiated by the space-based component using a first antenna pattern ofthe space-based component that spans a first geographic service area ofthe space-based component. The space-based component may also beconfigured to radiate the forward link control channel signal using asecond antenna pattern that spans a second geographic service area ofthe space-based component that may at least partially overlap with thefirst geographic area of the space-based component. The forward linkcontrol channel signal may be radiated using the second antenna patternafter the forward link control channel signal has been delayed by afirst delay value relative to the forward link control channel signalthat is radiated by the space-based component using the first antennapattern. The space-based component may also be configured to radiate theforward link control channel signal using a third antenna pattern thatspans a third geographic service area of the space-based component thatmay at least partially overlap with the first and/or second geographicarea of the space-based component. The forward link control channelsignal may be radiated using the third antenna pattern after it has beendelayed by a second delay value relative to the forward link controlchannel signal that is radiated by the space-based component using thefirst antenna pattern.

More generally, the space-based component may also be configured toradiate the forward link control channel signal using an Nth antennapattern that spans an Nth geographic service area of the space-basedcomponent that may at least partially overlap with the first, second,third, . . . , and/or (N-1)th geographic area of the space-basedcomponent. The forward link control channel signal may be radiated usingthe Nth antenna pattern after it has been delayed by a respective(N-1)th delay value relative to the forward link control channel signalthat is radiated by the space-based component using the first antennapattern. In some embodiments of the invention, the delay values (firstthrough (N-1)th) may be substantially predetermined and/or may besubstantially distinct. Furthermore, the N components of a signal thatmay be radiated by the space-based component over N respective antennapatterns may be radiated at N respective power levels that may bedifferent therebetween. The choice of the N respective power levels may,in accordance with some embodiments of the invention, be chosen based ona geographic position of a radioterminal that is to receive and processthe N components of the signal and/or in accordance with N respectivegain values, in the direction of the radioterminal, associated with Nrespective space-based component antenna patterns that are used toradiate the N respective power levels. The N respective power levels mayalso be evaluated subject to a constraint imposed on an aggregatespace-based component power to be used in radiating the N components ofthe signal over the N respective antenna patterns using the N respectivepower levels. In some embodiments, the radioterminal may also beconfigured to provide information to the space-based component and/or toa gateway of the space-based component (via a return link control and/ortraffic channel) to aid in determining an optimum or near optimum choiceof the N respective power levels.

Accordingly, a device that, is configured to receive and process theforward link control channel signal (or any other forward link signalthat is radiated by the space-based component in accordance with theprinciples disclosed hereinabove) may include a receiver element that isconfigured to increase or maximize a measure of desired signal to noiseand/or interference ratio by receiving and processing the signalradiated by the space-based component by the first antenna pattern andat least one delayed version thereof that is radiated by the space-basedcomponent by an antenna pattern other than the first. In someembodiments of the invention, the receiver element is a Rake receiverelement and/or a transversal filter receiver element, as will berecognized by those skilled in the art. Alternatively or in combinationwith the above, each one of the N forward link signal components that isradiated by the space-based component may be provided with a uniquecharacteristic (e.g., a unique pilot signal, bit sequence, mid-amble,pre-amble and/or spreading code) that a receiving device (such as aradioterminal) may process to achieve a maximal ratio combining (amaximum or near maximum of a desired signal to noise and/or interferencepower ratio) with respect to two or more of the forward link signalcomponents that are radiated by the space-based component over two ormore respective antenna patterns thereof and received at the receivingdevice.

It will be understood that any antenna pattern of the space-basedcomponent may be a first antenna pattern of the space-based component.It will also be understood that the space-based component may include aplurality of first antenna patterns and that each forward link antennapattern of the space-based component may be a first antenna pattern ofthe space-based component. In accordance with some embodiments of theinvention, a plurality of first antenna patterns associated with aspace-based component may be a number of first antenna patterns that isequal to, or is less than, a total number of antenna patterns associatedwith the space-based component. The total number of antenna patternsassociated with the space-based component may be, in some embodiments ofthe invention, a total number of beams/cells and/or antenna feed elementantenna patterns associated with the space-based component (such as atotal number of forward service link beams/cells and/or forward servicelink antenna feed element antenna patterns associated with thespace-based component). In some embodiments of the invention, at leastsome, and in some embodiments all, of the first antenna patterns of thespace-based component are associated with a neighboring/adjacent second,third, . . . and/or Nth antenna pattern, that, as described earlier,radiate respective second, third, . . . and/or Nth delayed versions ofan associated forward link signal and/or respective versions, includingunique characteristics, of the forward link signal. In some embodiments,the unique characteristics may include different code(s) and/ordifferent bit sequence(s) compared to a code and/or a bit sequence ofthe associated forward link signal. It will be understood that thetechniques described above relative to the forward link control channelsignal may be applied to any forward link control channel signal and/orany forward link traffic channel signal.

In some embodiments of the invention, at least one forward linkcommunications channel and/or at least one return link communicationschannel may be used preferentially for space-based communications and/ormay be reserved and used for space-based communications only, while oneor more forward link communications channels and/or one or more returnlink communications channels may be used for space-based and terrestrialcommunications and/or preferentially for terrestrial communications.Accordingly, the at least one forward link communications channel and/orthe at least one return link communications channel that may be reservedand used for space-based communications only and/or preferentially usedfor space-based communications, may be used to provide space-basedcommunications in geographic areas that are geographically proximate tosystem elements (ancillary terrestrial components) providing terrestrialcommunications using/reusing at least some frequencies of thespace-based component to thereby reduce or avoid interference that mayotherwise be caused by the terrestrial communications to the space-basedcommunications. Accordingly, a communications device that is engaged interrestrial-mode communications and is at a geographic distance that issubstantially at or beyond an edge of a geographic service area of thesystem elements providing terrestrial communications may be transferredto space-based-mode communications using the at least one forward linkcommunications channel and/or the at least one return linkcommunications channel that are/is reserved and used for space-basedcommunications only and/or is preferentially used for space-basedcommunications. It will be understood that the at least one forward linkcommunications channel and/or the at least one return linkcommunications channel that are/is reserved and used for space-basedcommunications only and/or preferentially used for space-basedcommunications, may also be used to provide space-based communicationsin geographic areas that are geographically distant to system elementsproviding terrestrial communications.

Embodiments of the invention may provide systems and methods forreducing Multiple Access Interference (MAI) and other (non-MAI)co-channel interference in a signal received by a space-based component.As noted above, co-channel interference may be generated by terrestrialuse/reuse of at least some of the satellite-band (space-based componentband) frequencies by an Ancillary Terrestrial Network (ATN) includinginfrastructure transmitters, such as, for example, base stationtransmitters and transmitters of user devices.

Modern satellites may use an antenna system including multiple receivingantenna feed elements to form a plurality of service area spot-beams (orantenna patterns). The antenna system may include a large number (L) ofantenna feed elements that may be physically arranged in a twodimensional array. Electromagnetic signals transmitted by user devices(e.g., radioterminals) and/or other transmitters are received by each ofthe L antenna feed elements. The electromagnetic signal received at thelth antenna feed element is referred to as y_(l). The collection ofsignals received at the L antenna feed elements is referred tocollectively as y_(L).

A received electromagnetic signal may be represented by a complex value(i.e., a value having both a real component and an imaginary component).Thus, a received electromagnetic signal may be referred to as a “complexsignal” and may be analyzed and manipulated using tools of mathematicsrelating, but not limited, to complex-valued quantities such asconstants, variables, functions, vectors and/or matrices.

In an antenna system, a collection of L complex weights (w_(L)) may beapplied to a received signal; that is, a complex weight w_(l) may beapplied to the signal y_(l) received at each of L feed elements of theantenna system. The complex weight applied to a signal y_(l) received atone feed element may be the same as, or different from, the complexweight y_(l) applied to a signal received at a different feed element.By making appropriate choices for the complex weights, the signalsreceived at each of the L feed elements may combine substantiallyconstructively or substantially destructively with each other dependingon respective azimuth and elevation values from which the signals arereceived relative to the orientation of the antenna. In general, eachset of complex weights may be chosen such that a signal power arrivingfrom a desired direction (azimuth/elevation combination) is maximized,or nearly maximized, at a receiver while a power of one or more signalsarriving at the receiver from one or more respective directions thatdiffer from the desired direction is suppressed. Thus, for example,applying a first set of L complex weights to the signals received by theL antenna feed elements may cause the antenna to be relativelyresponsive to signals received from around a first azimuth/elevationcombination and relatively unresponsive to signals received from otherazimuth/elevation combinations. A second set of L complex weights maycause the antenna to be relatively responsive to signals received fromaround a second azimuth/elevation combination and relativelyunresponsive to signals received from other azimuth/elevationcombinations, and so on.

By choosing appropriate combinations of L complex weights, an antennamay be configured to selectively receive signals from one or moreoverlapping or non-overlapping service areas, each of which isilluminated by a spot beam defined by a unique set of complex weights.Accordingly, “spot beam” refers to the area around a particularazimuth/elevation combination to which the antenna is responsive basedon a given set of L complex weights. A spot beam may therefore define ageographic region. The process of selecting appropriate complex weightsin order to define a spot beam having a desired response around aparticular azimuth/elevation combination is known as “beam forming.”

In some satellite systems such as Thuraya and Inmarsat-4, the signalsprovided by the satellite's receiving antenna feed elements aredigitally processed at the satellite by applying the complex weights tothe received complex signals and then forming linear combinations of thesignals in the manner described above. In other systems, however, thesignals received at the receiving antenna feed elements may betransported to a terrestrial satellite gateway via one or more satellitefeeder links, and processed at the satellite gateway in accordance withone or more performance criteria. This is referred to as ground-basedbeam forming.

In order to reduce co-channel interference, systems and/or methodsaccording to some embodiments of the invention may restrict the use ofavailable frequency bands such that a frequency band employed forsatellite communications within a particular satellite cell may not beemployed by elements of the ATN (e.g. fixed and/or mobile transmitters)located within the satellite cell. However, in order to increase theutilization of available bandwidth, the frequency band used forsatellite communications within a particular satellite cell may bespatially re-used outside the satellite cell. Signals transmitted overthe ATN using such re-used frequencies outside the satellite cell (i.e.outside the spot beam) may nevertheless be received as co-channelinterference by the satellite along with the intended satellitecommunications from within the satellite cell (i.e. inside the spotbeam). Such interference is referred to herein as ATN-induced orATC-induced co-channel interference. In some embodiments according tothe invention, however, frequencies used for satellite communicationswithin a particular cell may be terrestrially reused with additionalinterference reduction techniques, such as, for example, theinterference reduction techniques discussed in U.S. Pat. No. 6,684,057.

In some embodiments of the invention, pilot signals received at thesatellite's receiving antenna feed elements are used to perform adaptivebeam-forming to mitigate ATN-induced co-channel interference and/orinter-beam co-channel interference. Then, operating on thereduced-interference samples, an interference reducer removes at leastsome intra-beam MAI using multi-user detection. The space processing(beam-forming) may be performed in advance of the time processing(multi-user detection) in some embodiments, because it may be difficult(if not impossible) to perform effective signal detection without firstreducing the ATN-induced co-channel interference, which may beoverwhelming. In some embodiments, the adaptive beam-former uses apriori knowledge of pilot signals transmitted by the satellite userterminals, for example the pilot signal of the cdma2000 return linkwaveform. Following the beam-forming, the pilot signals are used toestimate multi-user channels. The detector may be a maximum likelihooddetector in some embodiments.

A single user interference reducing detector 200 including an adaptivebeam former 14 and an interference reducer 16 according to someembodiments of the invention is illustrated in FIGS. 2A-2C. As shown inFIG. 2A, a beam former 14 receives a vector y_(L) of L input signalsreceived at L feed elements of an antenna (not shown). The beam former14 also receives and/or has stored a vector of K pilot signal spreadingcodes p_(K). The vector p_(K) of pilot signal spreading codes includesone pilot signal spreading code for each of K multiple-accesstransmitters (i.e. satellite users) which transmit multiple-accesssignals to a satellite or space-based component (SBC) (not shown). Thus,the beam former has a priori knowledge of both the pilot signal and thepilot signal spreading codes with which the known pilot signal istransmitted by each of the K transmitters. This a priori knowledge isused both to locate (in time) the pilot signals as well as to reduce theinterference affecting the information signals transmitted by each ofthe K users (transmitters). The beam former 14 also receives, as aninput, delay information τ_(k) for each of the K transmitters that isprovided by a pilot searcher 12.

The beam former 14 generates an L×K matrix Ŵ_(K,L) of complex weights ŵ.That is, the beam former generates a vector ŵ_(K) of L complex weightsfor each of the K transmitters. As discussed above, each complex weightvector ŵ_(K) defines a set of complex weights which, when applied to theset of L signals received by the L antenna feed elements, forms a beamwhich reduces co-channel interference in the received pilot signals. Forexample, the weight vector Ŵ₁ defines a set of L weights which, whenapplied to the set of L signals received by the L antenna feed elements,forms a beam which reduces interference in the pilot signal receivedfrom the first transmitter, and so on. In some embodiments, the complexweight vector ŵ_(K) defines a set of complex weights which, when appliedto the set of L signals received by the L antenna feed elements, forms abeam which minimizes co-channel interference in the kth received pilotsignal. In some embodiments, the beam former 12 may use a Least MeanSquared Error (LMSE) algorithm to determine a set of complex weightswhich minimize co-channel interference in the received pilot signals.

The matrix Ŵ_(K,L) of complex weights is provided to the interferencereducer 16 along with the signals y_(L) received at the L feed elementsof the antenna. Interference reducer 16 uses the matrix Ŵ_(K,L) ofcomplex weights provided by the beam former 14 to generate Y_(K), a setof K signals (one for each of the K transmitters) having reducedinterference. Based on the values of the signals Y_(K), a “slicer” 18(e.g., a decision stage) generates an estimate {circumflex over (b)}_(K)of the bits transmitted by each of the K transmitters. In theembodiments of FIG. 2A, the beam former 14 and the interference reducer16 may be substantially similar, in that both reduce interference.However, beam former 14 is an autonomous element in that it derives aset of coefficients for reducing interference, by processing at leastone pilot signal and/or at least one information signal, whereas theinterference reducer is not an autonomous element in that it does notderive coefficients; instead, the interference reducer 16 usescoefficients provided by beam former 14 to reduce interference. It isunderstood, however, that in some embodiments of the invention, theinterference reducer 16 may also be configured to derive coefficients,by processing one or more pilot signals and/or one or more informationsignals, instead of receiving coefficients from the beam former 14 or incombination with receiving coefficients from beam former 14.

A beam former 14, according to some embodiments of the invention, isshown in more detail in FIG. 2B. As illustrated therein, a beam former14 may include an array 20 of K pilot signal estimators per feedelement. The beam former 14 may be configured to receive the L receivedsignals y_(L), K pilot signal spreading codes p_(K) and K delay timesτ_(k). In some embodiments, the beam former 14 may contain the pilotsignal spreading codes and may also be configured to determine the Kdelay times. As used herein an “estimator” may include a de-spreader andan integrator. The de-spreader may perform the function of de-spreadinga spread-spectrum signal by multiplying (correlating) thespread-spectrum signal with a spreading code that has been used by atransmitter of the spread-spectrum signal, and the integrator mayintegrate power of the de-spread spread-spectrum signal over a timeinterval to derive a measure of energy of the de-spread spread-spectrumsignal. The array 20 of pilot signal estimators generates a matrix ofL×K pilot signal estimates. That is, the array 20 of pilot signalestimators generates a vector of K pilot signal estimates (one for eachof the K received pilot signals) for each of the L antenna feedelements. A spatial combiner 22 combines the L×K pilot signal estimatesusing an initial set of postulated weights Ŵ_(K,L) and generates avector of K pilot signal estimates {circumflex over (d)}_(K) ^((p)). Anerror detector 24 compares the pilot signal estimates with the knownquantities associated with the pilot signals and generates an errorvector e_(K) of K error signals, one for each of the K pilot signals.The error vector e_(K) is fed back to the spatial combiner 22, whichuses the error vector e_(K) to adjust the value of the postulatedweights Ŵ_(K,L) to a new value based at least on the value of the errorvector e_(K). In some embodiments, the weights may be adjusted until theerror vector e_(K) is minimized in a LMS error sense. Other algorithmsmay be employed to reduce or minimize the error vector. The process maybe repeated until the system converges on a solution of weights Ŵ_(K,L)that reduces or minimizes a measure of the error vector e_(K). Thesolution of weights that satisfies the desired criterion is thenprovided as an output matrix Ŵ_(K,L) by the beam former 14. It will beunderstood that processing to establish an optimum or near optimummatrix of weights may be conducted at the chip level of aspread-spectrum waveform. That is, instead of de-spreading aspread-spectrum waveform, integrating power of the de-spreadspread-spectrum waveform and deriving an error quantity based on thede-spread waveform and a measure of energy thereof, a chip-level errorquantity may be derived by comparing a level of a chip of a receivedspread-spectrum waveform with a level of a reference (e.g., a level of achip of an ideal version of the received spread-spectrum waveform). Assuch, at least some function(s) of pilot signal estimators 20 may atleast partially be eliminated, and the spatial combiner 22 and/orinterference reducer 16 may be configured to operate on chip-level(before de-spreading) signals, as will be recognized by those skilled inthe art. In such embodiments, a de-spreader may be provided following abeam former and/or interference reducer.

An interference reducer 16 according to some embodiments of theinvention is illustrated in greater detail in FIG. 2C. As illustratedtherein, an interference reducer 16 may include an array 26 of K trafficsignal correlators (de-spreaders) per feed element. That is, theinterference reducer 16 may include L×K traffic signal correlators whichgenerate an L×K matrix Z_(K,L) ^((s)) of traffic signal estimates whichare provided to a spatial combiner 28. Using the matrix of weightsŴ_(K,L) of the beam former 14, the spatial combiner 28 forms a linearcombination of the L×K traffic signal estimates Z_(K,L) ^((s)) togenerate a set Y_(K) of K (de-spread) received signals (one for each ofthe K transmitters) having reduced interference. As stated earlier,those skilled in the art will recognize that the interference reducer 16may be configured to operate on chip-level (before de-spreading)signals. In such embodiments, a de-spreader may be provided followingthe interference reducer and at least some of the functions performed bytraffic signal correlators 26 may not be required.

As discussed above, a bit slicer 18 may be used to generate bitestimates {circumflex over (b)}_(K) from the set Y_(K) of K receivedsignals. In some embodiments, the slicer 18 may be implemented as acomparator whose output is sampled at times based on the time delayτ_(k) for each of the K transmitters.

An interference reducing detector 200 is illustrated in more detail inFIG. 11. As illustrated therein, the interference reducing detector 200includes L feed elements 1105 which supply L signals to the detector200. The L signals may be received, for example, by L antenna feedelements of an antenna (not shown). The L received signals are suppliedto a bank of K pilot signal correlators 1120 which correlate thereceived signals with the known pilot signal spreading codes p_(K).Timing information for the pilot signal correlators 1120 is provided bythe K pilot searchers 1112. The de-correlated pilot signals areintegrated over Q periods (such as Q periods of an information symbol)by integrators 1125 and spatially combined by combiners 1122 to generateK received pilot signal estimates. The pilot signal estimates arecompared by error detectors 1124 with known values relating to pilotsignals to generate K pilot signal error vector signals e_(K), which arefed back to the spatial combiners 1122 and used to improve the weights.It will be understood that the L feed elements 1105 may be located at aspace-based component and at least some other element of theinterference reducing detector 200 may be located distant from thespace-based component.

The L signals (y_(L)) supplied by the L feed elements 1105 are alsoprovided to a bank of K traffic signal correlators 1126 which de-spreadthe signals based on known traffic signal spreading codes s_(K). Thede-spread information signals are then combined by a spatial combiner1128 which uses the weights generated by the spatial combiners 1122 togenerate K received information signals Y_(K). Each of the K receivedinformation signals is then processed by a slicer 1118 to generate bitestimates (channel bit estimates).

An interference reducing detector 300 configured to perform co-channelinterference reduction and multiple access interference reductionaccording to further embodiments of the invention is illustrated inFIGS. 3A-3B. Some elements of interference reducing detector 300 aresimilar to respective elements of the interference reducing detector 200illustrated in FIG. 1A. That is, the detector 300 includes a pilotsearcher 12 and a beam former 14. As in the detector 200, the pilotsearcher 12 generates delay information τ_(k) for each of the Ktransmitters and provides the delay information to a beam former 14along with a vector y_(L) of L input signals received at L feed elementsof an antenna. The beam former 14 also receives and/or has stored avector of K pilot signal spreading codes and generates an L×K matrixŴ_(K,L) of complex weights ŵ. The complex weights ŵ areadaptively/recursively improved by the beam former 14 according to analgorithm such as, for example, LMSE described above.

The matrix Ŵ_(K,L) of complex weights is provided to an interferencereducer 30 (which may be similar to the interference reducer 16) alongwith the received signals Y_(L) from each of the L feed elements of theantenna. In the system 300 the interference reducer 30 provides ade-spread signal Y_(K), for each of the K signals, and also a chip-levelsignal r_(K) for each of the K user signals. The chip-level signal isused by a channel estimator 32 to generate channel estimates Â_(K,K) foreach of the K user signals received at the antenna feed elements. Thechannel estimates Â_(K,K) are provided along with K bit estimates{circumflex over (b)}_(K) generated by a slicer 31 and the K chip-levelsignals r_(K) to a Sequential ATC and MAI Interference Cancellation(SAMIC) detector 34. In accordance with some embodiments of theinvention, the SAMIC detector 34 generates a MAI-cancelled version ofthe chip-level signals r_(K). The chip-level signals {tilde over(r)}_(K) generated by the SAMIC detector 34 are then processed by atraffic signal de-spreader 36 which has a priori knowledge of thespreading codes s_(K) used by each of the K transmitters to generate avector of K MAI-reduced bit estimates {tilde over ({circumflex over(b)}_(K).

An interference reducer 30 according to some embodiments of theinvention is illustrated in FIG. 3B. As shown therein, an interferencereducer 30 may include a spatial combiner 38 which is configured toreceive the received signal vector y_(L) along with the matrix Ŵ_(K,L)of complex weights generated by the beam former 14. The spatial combiner38 forms linear combinations of the input signal vector Y_(L) valuesusing the complex weights Ŵ_(K,L) to generate a vector of K receivedchip-level signals r_(K), which is provided as a first output of theinterference reducer 30. The interference reducer 30 may also include atraffic signal de-spreader 40 which is configured to de-spread thereceived information signal r_(K) to generate a vector of K receivedsignals Y_(K), which is provided as a second output of the interferencereducer 30. The received signals Y_(K) may be processed by the slicer 31(FIG. 3A) to provide k bit estimates L_(K).

An interference reducing detector 300 is illustrated in more detail inFIG. 12. As illustrated therein, as in the interference reducingdetector illustrated in FIG. 11, the interference reducing detector 300includes L feed elements 1105 which supply L signals to the detector300. The L signals may be received, for example, by L antenna feedelements of an antenna (not shown). The L received signals are suppliedto a bank of K pilot signal correlators (de-spreaders) 1120 whichcorrelate the received signals with the known pilot signal spreadingcodes p_(K). Timing information for the pilot signal correlators 1120 isprovided by the K pilot searchers 1112. The de-correlated pilot signalsare integrated over Q periods by integrators 1125 and spatially combinedby combiners 1122 to generate K received pilot signal estimates. Thepilot signal estimates are compared by error detectors 1124 with knownpilot signal values to generate K pilot signal error vector signalse_(K), which are fed back to the spatial combiners 1122 and used toimprove the weights.

The L signals (y_(L)) supplied by the L feed elements 1105 are alsoprovided to a bank of K spatial combiners 1238 which use the weightsgenerated by the spatial combiners 1122 to generate K received chiplevel signals r_(K) having reduced co-channel interference. The Kinterference reduced chip level signals are then processed by K trafficsignal correlators 1240 and slicers 1218 to generate K bit estimates{circumflex over (b)}_(K) for the K detected signals.

The K interference reduced chip level signals r_(K) are also provided toa bank of channel estimators 1232 which generate K channel estimatesα_(K) for each of the K signals. The channel estimates α_(K,K) areprovided along with the chip level signals r_(K) and the bit estimates{circumflex over (b)}_(K) generated by the slicers 1218 to a bank ofSAMIC detectors 1234 which perform multiple access interferencecancellation on the interference reduced chip level signals r_(K) usingthe channel estimates α_(K,K) and the bit estimates {circumflex over(b)}_(K). The resulting MAI reduced received chip level signals {tildeover (r)}_(K) are then processed by a bank of K traffic signalcorrelator/slicers 1246 which generate MAI-reduced bit estimates {tildeover ({circumflex over (b)}_(K).

In some embodiments of the invention, a second SAMIC detector may beemployed to further improve interference reduction. As illustrated inFIG. 4A, an interference reducing detector 400A configured to performco-channel interference reduction and multiple access interferencereduction according to further embodiments of the invention isillustrated. System 400A may include elements from system 300, namely, abeam former 14 which generates a matrix of complex weights Ŵ_(K,L) basedon analysis of received pilot signals, an interference reducer 30 whichis configured to generate bit estimates {circumflex over (b)}_(K) (viathe slicer 31) and received chip level signals r_(K), a first channelestimator 32 configured to generate channel estimates Â_(K,K) from thereceived chip level signals r_(K), and a first SAMIC detector 34configured to receive the bit estimates {circumflex over (b)}_(K), thechannel estimates Â_(K,K) and the received chip level signals r_(K), andgenerate preliminary interference reduced chip level signals {tilde over(r)}_(K). Interference-reduced bit estimates {tilde over ({circumflexover (b)}_(K) are generated by a first traffic signal de-spreader 36.

In addition to the first SAMIC detector 34, the system 400A furtherincludes a second channel estimator 42, a second SAMIC detector 44 and asecond traffic signal de-spreader 46. The second channel estimator 42receives the preliminary MAI-reduced chip level signals {tilde over(r)}_(K) and generates a matrix of second channel estimates Â′_(K,K).Since the second channel estimates are generated based on preliminaryMAI-reduced signals {tilde over (r)}_(K) generated by the first SAMICdetector 34, they may be more accurate estimates of the transmissionchannels. In the system 400A, the first traffic signal de-spreader 36generates MAI-reduced preliminary bit estimates {tilde over ({circumflexover (b)}_(K) which are provided to the second SAMIC detector 44 alongwith the second channel estimates Â_(K,K) generated by the secondchannel estimator 42. The second SAMIC detector 44 uses the MAI-reducedpreliminary bit estimates {tilde over ({circumflex over (b)}_(K) and thesecond channel estimates Â_(K,K) to generate second MAI-reduced chiplevel signals {tilde over ({tilde over (r)}_(K), which are thenprocessed by a second traffic signal de-spreader (correlator/slicer) 46to provide final MAI-reduced bit estimates

. It will be understood that the above procedure relating to the firstand second SAMIC stages may be repeated, in some embodiments, to provideadditional SAMIC stages.

Further embodiments of the invention are illustrated in FIG. 4B whichshows a detector 400B. In the detector 400B, multi-level interferencereduction using a SAMIC detector is illustrated. As shown therein, adetector 400B may include a single SAMIC detector 34. Multi-level SAMICdetection may be accomplished by feeding the MAI-reduced received chiplevel signal {tilde over (r)}_(K) generated by the SAMIC detector 34back to the channel estimator 32, and feeding back the MAI-reduced bitestimate {tilde over ({circumflex over (b)}_(K) generated by the trafficsignal de-spreader 36 back to the SAMIC detector 34. The MAI-reducedreceived chip level signal {tilde over (r)}_(K) may be fed back to thechannel estimator 32 one or more times, and the bit estimate signal{tilde over ({circumflex over (b)}_(K) generated by the traffic signalde-spreader 36 may be fed back to the SAMIC detector 34 one or moretimes. Each iteration of the feedback loop may generate a subsequentinterference reduced chip level signal {tilde over (r)}_(K) by the SAMICdetector 34.

Some embodiments of the invention are illustrated in FIGS. 5-8. Asillustrated in the embodiments of FIG. 5, in a process for single stageSAMIC detection, an array of signals is received via L feed elements(block 510). Co-channel interference reduction is performed on thereceived signals (block 520) to detect signals from K transmitters.Finally, SAMIC detection is performed on the K interference-reducedsignals to reduce multiple-access interference in the received signals(block 530).

Dual-stage SAMIC detection is illustrated in FIG. 6. As shown therein,an array of signals is received via L feed elements (block 610).Co-channel interference reduction is performed on the received signals(block 620) to detect signals from K transmitters. A first stage ofSAMIC detection is performed on the K interference-reduced signals toreduce multiple-access interference in the received signals (block 630).A second stage of SAMIC detection is then performed using theinterference-reduced signals as inputs to a second stage SAMIC detector(block 640). Accordingly, the second stage SAMIC detector usespreliminary bit estimates {tilde over ({circumflex over (b)}_(K) fromthe first stage SAMIC detector and second channel estimates Â′_(K,K) togenerate second MAI-reduced chip level signals {tilde over ({tilde over(r)}_(K) which are then processed to provide final (provided there areno additional SAMIC stages) MAI-reduced bit estimates

Multi-stage SAMIC detection is illustrated in the flowchart of FIG. 7.As in single- and dual-stage SAMIC detection, an array of signals isreceived via L feed elements (block 710), and co-channel interferencereduction is performed on the received signals (block 720) to detectsignals from K transmitters. SAMIC detection is performed on theMAI-reduced signals to provide interference-reduced bit estimates (block730). A bit error rate (BER) is calculated and compared to a threshold(block 740). If the calculated bit error rate is acceptable, thecalculated bit estimates are used. If not, a subsequent stage of SAMICdetection is performed using the interference-reduced bit estimates asinputs. The process may repeat until a predetermined exit criterion ismet. For example, the process may repeat until an acceptable BER isobtained, a maximum number of iterations has occurred, the BER hasconverged, or some other criterion is met.

Single stage SAMIC detection is illustrated in more detail in FIG. 8. Asshown therein, an array of signals is received via L feed elements of anantenna system (block 810). Timing information for each of the K usersis determined by a pilot searcher (block 820). Pilot spreading codes andsignal spreading codes for each of the K users are obtained (block 830).It will be understood that in some cases, the pilot spreading codesand/or signal spreading codes may be known in advance and need not bedynamically obtained. Furthermore, the pilot spreading codes and/orsignal spreading codes may be stored in an interference reducer, areceiver, and/or at a remote database. Thus, obtaining the spreadingcodes may include retrieving the spreading codes from a local and/orremote database.

Once the pilot signal spreading codes are known, pilot signal estimatesare obtained (block 840). In particular, K pilot signal estimates (onefor each of the K transmitters) may be obtained for each of the Lantenna feed elements. In some cases, the pilot signal estimates may beaveraged over Q periods in order to increase the signal-to-noise ratioof the pilot signals. The pilot signal estimates may be spatiallycombined to provide a single pilot signal estimate for each of the Ktransmitters. Based on the pilot signal estimates, optimum weightsŴ_(K,L) are determined (block 850). In some cases, the weights Ŵ_(K,L)may be selected to provide an LMS error of pilot signal estimates. Thecalculated weights are applied to the L received signals to obtain Kcomplex received chip level signals r_(K) (block 860), which are thende-spread using the known signal spreading codes (block 870).

Once the received chip level signals r_(K) are detected, bit estimatesare obtained (block 880). Channel estimates Â_(K,K) may also be obtainedfrom the received chip level signals r_(K) (block 890). MAI interferencereduction may then be performed using a SAMIC detector based on thereceived chip level signals r_(K), the interference-reduced bitestimates {circumflex over (b)}_(K), and the channel estimates Â_(K,K)(block 900). The resulting MAI-reduced chip level signals {tilde over(r)}_(K) may be used to obtain second bit estimates {tilde over({circumflex over (b)}_(K) (block 910).

Methods and systems according to some embodiments of the invention willnow be described in greater detail. The following description isorganized as follows. In Section 1, a system model and a problem ofinterest are formulated. A pilot-based Minimum Mean-Squared Error (MMSE)interference canceling single-user detector is then developed in Section2. In Section 3, a SAMIC multi-user detector according to someembodiments of the invention is presented. In Section 4, simulationresults are provided illustrating the performance of the interferencecancellation algorithm by using a representative satellite system designand an ATN footprint over CONUS.

1. System Model

In the satellite system model discussed herein, the satellite forwardlinks are assumed to form fixed spot beams. Each of the fixed forwardlink spot beams is analogous to a terrestrial cell, though much biggergeographically. A three-cell frequency reuse cluster size is assumed, asdepicted in FIG. 9. As shown in FIG. 9, a number of ATC towers may existwithin a spot beam. The ATCs and the wireless terminals communicatingtherewith may use frequencies of adjacent spot beams in order toincrease or maximize the isolation between the terrestrial and satellitereuse of the available satellite band frequencies. FIG. 9 also shows“exclusion” zones (dotted circles) inside of which the frequencies ofthe encircled satellite cell may not be made available to any ATCcontained therein. FIG. 9 also illustrates the typically largergeographic footprints of return-link satellite antenna feed elements.The signals provided to the satellite gateway by such return-linkantenna feed elements may be used to perform adaptive (return-link)signal processing comprising beam-forming, interference cancellation,channel estimation and multi-user detection.

The satellite communications channel is assumed to be Ricianflat-fading, however, other channel models may also be assumed. For thek^(th) return-link satellite user, the vector channel impulse responseacross L feed elements may be written as

h _(k)(τ,t)=a _(k)(θ_(k),φ_(k))β_(k)(t)δ(τ−τ_(k))  (1)

where

a _(k)(θ_(k),φ_(k))=[a _(k,1)(θ_(k),φ_(k)), . . . a_(k,L)(θ_(k),φ_(k))]^(T) εC ^(L×1)  (2)

is the satellite return-link antenna feed element complex responsevector for the k^(th) user located at elevation angle θ_(k) and azimuthangle φ_(k). A typical 3-D complex gain plot of a feed element is shownin FIG. 10. The quantity

β_(k)(t)=ρ_(k)exp{j(2πf _(k) t+ψ _(k))}  (3)

is the return-link path gain for the k^(th) user, f_(k) is the Dopplershift, ψ_(k) is a fixed phase shift, and τ_(k) is a time delay of thek^(th) user.

With the model of vector channel impulse response, for a generalmulti-user system with a total of K users, the data vector of the L feedelement output can be expressed as

$\begin{matrix}{{y(t)} = {{\sum\limits_{k = 1}^{K}{\left\lbrack {{g_{s}{b_{k}(t)}{s_{k}(t)}} + {g_{p}{p_{k}(t)}}} \right\rbrack*{h_{k}\left( {\tau,t} \right)}}} + (4)}} \\{{{{\sum\limits_{n = 1}^{N}{{a_{n}\left( {\theta_{n},\phi_{n}} \right)}g_{n}{v_{n}(t)}}} + {n(t)}} \in C^{L \times 1}}} \\{= \left. {\sum\limits_{k = 1}^{K}\begin{bmatrix}{{{a_{k}\left( {\theta_{k},\phi_{k}} \right)}{\beta_{k}(t)}g_{s}{b_{k}\left( {t - \tau_{k}} \right)}{s_{k}\left( {t - \tau_{k}} \right)}} +} \\{{a_{k}\left( {\theta_{k},\phi_{k}} \right)}{\beta_{k}(t)}g_{p}{p_{k}\left( {t - \tau_{k}} \right)}}\end{bmatrix}}\leftarrow{\left( {K\mspace{14mu} {users}} \right) +} \right.} \\{\left. {\sum\limits_{n = 1}^{N}{{a_{n}\left( {\theta_{n},\phi_{n}} \right)}g_{n}{v_{n}(t)}}}\leftarrow{\left( {N\; A\; T\; C\mspace{14mu} {interferers}} \right) +} \right.} \\{\left. {n(t)}\leftarrow\left( {{Gaussian}\mspace{14mu} {thermal}\mspace{14mu} {noise}} \right) \right.}\end{matrix}$

where b_(k)(t) and s_(k)(t) are the k^(th) user's information bit andspreading sequence, respectively, with M chips/bit; p_(k)(t) is thek^(th) user's pilot chip sequence; and g_(s) and g_(p) are theamplitudes of the traffic data signal and the pilot signal, respectively(same for all K users). The quantity v_(n)(t) denotes the aggregateinterference signal of the n^(th) ATC service area modeled as complexGaussian noise, and g_(n) is an associated amplitude. Finally,n(t)εC^(L×1) represents an additive complex Gaussian noise vector.

For the l^(th) antenna feed element, if matched filtering is performedon the received signal by correlating the received signal with the chipwaveform for each chip interval, the received signal in the l^(th)element can be written as:

$\begin{matrix}{y_{l} = {{{\sum\limits_{k = 1}^{K}\begin{bmatrix}{a_{k,l}\left( {\theta_{k},\phi_{k}} \right)} \\{\beta_{k}\begin{pmatrix}{{g_{s}b_{k}s_{k}} +} \\{g_{p}p_{k}}\end{pmatrix}}\end{bmatrix}} + {\sum\limits_{n = 1}^{N}{{a_{n,l}\left( {\theta_{n},\phi_{n}} \right)}g_{n}v_{n}}} + n_{l}} \in C^{m \times l}}} & (5)\end{matrix}$

where s_(k) and p_(k) are the chip matched-filter M-vectorscorresponding to s_(k)(t−T_(k)) and p_(k)(t−T_(k)) respectively. It isassumed that the spreading codes for the signal and pilot are normalizedto have unit energy: ∥s_(k)∥=1, ∥p_(k)∥=1, and that they are orthogonalfor a given user (i.e.,

s_(k),p_(k)

=0); v_(n) is the complex M-vector Gaussian noise corresponding to then^(th) ATC interference, and n_(l) is the complex M-vector correspondingto the Gaussian noise at the l^(th) antenna feed element.

By introducing some new matrix notation, Equation (5) may be rewrittenas:

y _(l) =SA _(l) bg _(s) +PA _(l)1_(K) g _(p) +VA _(l) ^((n))1_(N) g _(n)+n _(l)  (6)

where:

-   S=[s₁ s₂ . . . s_(K)]εC^(M×K)≡data spreading code matrix-   A_(l=diag{a) _(1,l)(θ₁,φ₁)β₁ . . .    a_(K,l)(θ_(K),φ_(K))β_(K)}εC^(K×K)≡l^(th) feed element/channel    matrix-   b=[b₁ . . . b_(K)]^(T)ε    ^(K×l)≡K-vector of data bits-   P=[p₁ p₂ . . . p_(K)]εC^(M×K)≡pilot spreading code matrix-   1_(u)=[1 . . . 1]^(T)ε    ^(u×1)≡u-vector of ones-   V=[v₁ v₂ . . . v_(N)]εC^(M×N)≡ATC interference matrix-   A_(l) ^((n))=diag{a_(1,l)(θ₁,φ₁) . . .    a_(N,l)(θ_(N),φ_(N))}εC^(N×N)≡l^(th) feed element matrix for N ATCs

The noise vector n_(l)εC^(M×1) is a zero-mean complex Gaussian vectorwhose distribution can be written in terms of real and imaginarycomponents:

$\begin{matrix}{{\left. \begin{bmatrix}{{Re}\left( n_{l} \right)} \\{{Im}\left( n_{l} \right)}\end{bmatrix} \right.\sim\eta}\left\{ {\begin{bmatrix}0_{M} \\0_{M}\end{bmatrix},{\sigma^{2}\begin{bmatrix}I_{M} & 0_{M \times M} \\0_{M \times M} & I_{M}\end{bmatrix}}} \right\}} & (7)\end{matrix}$

Real and imaginary components for matrices and vectors are defined asRe(X)=(X+X*)/2 and lm(X)=(X−X*)/2 where “*” denotes complex conjugate.

The ATC interference vector v_(n)εC^(M×l) (for the n^(th) ATC, n=1, 2, .. . N) is modeled as a zero-mean complex Gaussian vector. Assuming eachof all N ATCs has the same power (variance=λ²), the distribution of theATC interference vector may be written as:

$\begin{matrix}{{\left. \begin{bmatrix}{{Re}\left( v_{n} \right)} \\{{Im}\left( v_{n} \right)}\end{bmatrix} \right.\sim\eta}\left\{ {\begin{bmatrix}0_{M} \\0_{M}\end{bmatrix},{\lambda^{2}\begin{bmatrix}I_{M} & 0_{M \times M} \\0_{M \times M} & I_{M}\end{bmatrix}}} \right\}} & (8)\end{matrix}$

The problem of interest herein is to estimate b_(k) (k=1, 2, . . . K)from the y_(l) (l=1, 2, . . . L).

2. Pilot based MMSE Interference Cancellation

This section describes how estimates for the combining weights may beobtained subject to an error reducing criterion such as, for example, aminimum mean squared error (MMSE) criterion in the cdma2000 satellitereturn link according to some embodiments of the invention. Since theMMSE criterion is applied to the received signal with ATC interference,the resulting solution may be optimal for ATC interference cancellationin the sense of minimum mean squared error.

2.1 Pilot Spatial Channel MMSE Estimator

Let z_(l) ^((p)) be the K-complex vector output from a bank of K filtersmatched to users' delayed pilot signal p₁ p₂ . . . p_(K), whose input(y_(L)) is the received baseband signal at feed element l. The timingestimate for each of these users is assumed to be obtained though apilot searcher. For the l^(th) element, the K-complex vector output fromthe bank of K matched filters is the de-spread version of received pilotsignals, which is given by

z _(l) ^((p)) =P ^(H) y _(l) =R ^((p)) A _(l)1_(K) g _(p) +R ^((ps)) A_(l) bg _(s) +R ^((pv)) A _(n)1_(N) g _(n) +P ^(H) n _(l) εC ^(K×1)  (9)

where (•)^(H) denotes the complex conjugate transpose, and

-   R^((p))=P^(H)PεC^(K×K)≡pilot correlation matrix with ones along the    main diagonal-   R^((ps))=P^(H)SεC^(K×K)≡pilot/signal cross-correlation matrix with    zeros along main diagonal-   R^((pv))=P^(H)VεC^(K×N)≡pilot/ATC cross-correlation matrix

From Equation (9), the normalized de-spread pilot channel output vectormay be derived as:

$\begin{matrix}{d_{1}^{(p)} = {\frac{z_{1}^{(p)}}{g_{p}} = {\underset{{({desired})}}{A\; 1_{K}} + {\left( {R^{(p)} - I_{k}} \right)\underset{({MAI})}{A\; 1_{K}}} + {R^{({ps})}{Ab}\; \frac{g_{s}}{g_{p}}} + \underset{{{A\; T\; C\mspace{14mu} {interference}}{Noise}}}{{R^{({pv})}A_{n}1_{N}\frac{g_{n}}{g_{p}}} + {\frac{1}{g_{p}}P^{H}n_{1}}}}}} & (10)\end{matrix}$

Assuming the feed element and channel responses do not change over aperiod of Q symbols, the pilot estimate can be improved by averaging Qsuccessive instances of d_(l) ^((p)). In the simulation study, thefollowing approximation for the averaged estimate using long codes isused:

$\begin{matrix}\begin{matrix}{{\hat{d}}_{1}^{(p)} = {\frac{1}{{Qg}_{p}}{\sum\limits_{q = 1}^{Q}z_{1,q}^{(p)}}}} \\{= {{A_{l}1_{K}} + {\frac{1}{\sqrt{Q}}\left( {R^{(p)} - I_{k}} \right)A_{l}1_{K}} + {\frac{1}{\sqrt{Q}}R^{({ps})}A_{l}b\; \frac{g_{s}}{g_{p}}} +}} \\{{{\frac{1}{\sqrt{Q}}R^{({pv})}A_{n}1_{N}\frac{g_{n}}{g_{p}}} + {\overset{\_}{n}}_{1}}}\end{matrix} & (11)\end{matrix}$

where the complex Gaussian noise term has distribution given as:

n _(l)˜η{0_(K),g_(p) ⁽⁻²⁾σ²R^((p))/Q}.

From Equation (11) it may be seen that averaging the pilot signalestimates over a window of Q symbols reduces the variances of MAI, ATCinterference and noise by a factor of Q. Another interesting aspect isthat if short codes are used, there would be no 1/√{square root over(Q)} factor for the pilot interference term (R^((p))−I_(k))Al_(K)(because the values remain constant over window. Therefore, the pilotestimates suffer in the long code case. But this potential disadvantagecan be removed by introducing the 1/√{square root over (Q)} factor withknown pilot sequence.

Since the pilot signal estimates contain ATC interference and MAI, thenext issue is to mitigate ATC interference by taking advantage ofmultiple feed elements and known pilot signals (removing MAI will betaken care of later). If the estimate of the k^(th) user's pilot vectoracross L feed elements is defined as

y _(k) ^((p)) =[{circumflex over (d)} ₁ ^((p))(k){circumflex over (d)} ₂^((p))(k) . . . {circumflex over (d)} _(L) ^((p))(k)]^(T) εC^(L×1)  (12)

where {circumflex over (d)}₁ ^((p)) is defined in (10), then thepilot-based MMSE interference cancellation criterion may be derived.

The MMSE criterion attempts to minimize the difference between theoutput of the beam former and the desired user's response. Morespecifically, for the k^(th) user, the weight is given as:

$\begin{matrix}\begin{matrix}{w_{k} = {\underset{w_{k}}{argmin}\left\{ {J\left( w_{k} \right)} \right\}}} \\{= {\underset{w_{k}}{argmin}\left\{ {E\left\lbrack {{{w_{k}^{H}y_{k}^{(p)}} - d_{k}}}^{2} \right\rbrack} \right.}} \\{= {\sigma_{d}^{2} - {w_{k}^{H}r_{k}} - {r_{k}^{H}w_{k}} + {w_{k}^{H}R_{k}w_{k}}}}\end{matrix} & (13)\end{matrix}$

where y_(k) ^((p)) is the array output, d_(k) is the desired response,σ_(d) ²=E{|d_(k)|²},

R _(k) =E[y _(k) ^((p))(y _(k) ^((p)))^(H)]  (14)

is the spatial covariance matrix for the k^(th) user and

r _(k) =E[y _(k) ^((p)) d _(k)*]  (15)

is the cross-correlation vector between the input data and the desiredd_(k). The optimal solution that minimizes the MSE is given by

w _(k) =R _(k) ⁻¹ r _(k)  (16)

The MMSE interference canceller can be implemented, for example, with acomputationally efficient Least Mean Square (LMS) adaptive algorithm.The gradient vector of the error surface is

$\begin{matrix}{\nabla_{w_{k}{(n)}}{= {{{\frac{\partial}{\partial w_{k}}{J\left( w_{k} \right)}}_{w_{k} = {w_{k}{(n)}}}} = {{{- 2}r_{k}} + {2R_{k}{w_{k}(n)}}}}}} & (17)\end{matrix}$

Adjusting the weight vector in the steepest descent gradient directionleads to an LMS adaptive algorithm that is given by:

w _(k)(n+1)=w _(k)(n)+μy _(k) ^((p))(n)e _(k)*(n)εC ^(L×1)  (18)

where e_(k)(n)=d_(k)(n)−w_(k) ^(H)(n)y_(k) ^((p))(n) is the errorsignal, and μ is the step-size coefficient that should be chosen as

$0 < \mu < {\frac{1}{{Trace}\left\lbrack R_{k} \right\rbrack}.}$

The convergence rate is governed by the eigenvalue spread of R_(k).

Applying the weight ŵ_(k) to the k^(th) user's pilot vector y_(k) ^((p))yields an estimate of the pilot symbol after adaptive beam forming forATC interference cancellation as follows:

$\begin{matrix}\begin{matrix}{{\hat{p}}_{{({symb})}_{k}} = {{\hat{w}}_{k}^{H}y_{k}^{(p)}}} \\{= {{\hat{w}}_{k}^{H}\left\lbrack {{{\hat{d}}_{1}^{(p)}(k)}{{\hat{d}}_{2}^{(p)}(k)}\mspace{14mu} \ldots \mspace{14mu} {{\hat{d}}_{L}^{(p)}(k)}} \right\rbrack}^{T}} \\{= {\sum\limits_{l = 1}^{L}{{{\hat{w}}_{k}^{H}(l)}{{\hat{d}}_{l}^{(p)}(k)}}}}\end{matrix} & (19)\end{matrix}$

2.2. Single-User Traffic Signal Detector

The resulting weight vector ŵ_(k) for the k^(th) user may represent aspatial MMSE solution that reduces the ATC co-channel interference plusthermal noise based on the pilot channel. Since the pilot signal andtraffic data signal are received through the same feed element andpropagation channel, the estimated weight ŵ_(k) may be applied to thetraffic data channel to perform the interference cancellation as well.As shown in FIG. 11, the interference reducer is a generalization of abank of K correlators 1126 (one for each user) per feed element,followed by a spatial combiner 1128 for interference cancellation.

The K correlators are matched to the spreading code s₁ s₂ . . . s_(K).At feed element l, the resulting K vector output is given as:

z _(l) ^((s)) =S ^(H) y _(l) =R ^((s)) A _(l) bg _(s) +R ^((sp)) A_(l)1_(K) g _(p) +R ^((sv)) A _(l) ^((n))1_(N) g _(n) +S ^(H) n _(l) εC^(K×1)  (20)

where

-   R^((s))=S^(H)SεC^(K×K)≡traffic signal correlation matrix with ones    along the main diagonal-   R^((sp))=S^(H)PεC^(K×K)≡traffic signal and pilot cross-correlation    matrix with zeros along the main diagonal-   R^((sv))=S^(H)VεC^(K×N)≡traffic signal and ATC cross-correlation    matrix

The correlator output for the k^(th) user at feed element l is weightedby (ŵ_(k) ^(H))_(l). By defining the interference cancellation weightingmatrix

Ŵ _(l)=diag{(ŵ ₁)_(l) (ŵ ₂)_(l) . . . (ŵ _(K))_(l) }εC ^(K×K)  (21)

where (•)_(l) denotes the l^(th) element of a vector, the weighted andcombined output for all K users may be derived as follows:

$\begin{matrix}\begin{matrix}{Y = {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}z_{l}^{(s)}}} \right)}} \\{= {{Re}\begin{pmatrix}{{\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{(s)}A_{l}{bg}_{s}}} + {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{({sp})}A_{l}1_{K}g_{p}}} +} \\{{\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{({sv})}A_{l}^{(n)}1_{N}g_{n}}} + {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}S^{H}n_{l}}}}\end{pmatrix}}}\end{matrix} & (22)\end{matrix}$

To simplify the expression, the following definitions may be provided:

$\begin{matrix}{X^{(s)} \equiv {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{(s)}A_{l}}} \right)}} & (23) \\{X^{({sp})} \equiv {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{({sp})}A_{l}}} \right)}} & (24) \\{X^{({sv})} \equiv {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{({sv})}A_{l}^{(n)}}} \right)}} & (25) \\{n \equiv {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}S^{H}n_{l}}} \right)}} & (26)\end{matrix}$

Then, equation (22) can be rewritten as

$\begin{matrix}\begin{matrix}{Y = {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}z_{l}^{(s)}}} \right)}} \\{= {{{X^{(s)}{bg}_{s}} + {X^{({sp})}1_{K}g_{p}} + {X^{({sv})}1_{N}g_{n}} + n} \in ^{K \times 1}}}\end{matrix} & (27)\end{matrix}$

and the single-user data symbol estimate for the k^(th) user is given bythe algebraic sign of the k^(th) component as follows:

{circumflex over (b)} _(k)=sgn(Y _(k))  (28)

Note that n˜η(0_(k),σ²{circumflex over (X)}^((n))), where

${{\hat{X}}^{(n)} \equiv {{Re}\left( {\sum\limits_{l = 1}^{L}{{\hat{W}}_{l}^{H}R^{(s)}{\hat{W}}_{l}}} \right)}},$

and the bit error rate (BER) for the k^(th) user is given by

$\begin{matrix}{{P_{k}(\sigma)} = {Q\left( \frac{\left( {{X^{(s)}I_{k}{bg}_{s}} + {X^{({sp})}1_{K}g_{p}}} \right)_{k}}{{g_{n}\left( {X^{({sv})}1_{N}} \right)}_{k} + {\sigma \sqrt{\left( {\hat{X}}^{(n)} \right)_{k}}}} \right)}} & (29)\end{matrix}$

As can be seen, the BER is dependent on other user's bits, number andlevels of ATC interference, the feed element/channel coefficients, andinterference cancellation weight estimates.

The single-user detector that has been derived above is an ATCinterference cancellation version of a single-user detector. For casesinvolving more than one users (K>1), the single-user detector willgenerally suffer from multiple access interference from other users.Mathematically, this MAI results in non-zero components off the maindiagonal of the cross-correlation matrix R^((s)). Further embodiments ofthe invention, as derived below, provide a multi-user detectionalgorithm to remove MAI by taking advantage of formed-beam/channelestimates that become available from the pilot channel after thecancellation of ATC-induced co-channel interference.

3. Multi-User Detection in Conjunction with ATC InterferenceCancellation

ATC induced interference includes inter-beam, co-channel interferencethat may be effectively addressed by an adaptive interference reducingdetector. Unlike ATC interference, multiple access interference (MAI)includes intra-beam interference that may not be removed effectively byspatial-only processing techniques. Some embodiments of the inventionprovide algorithms for the efficient reduction of MAI after ATCinterference reduction. In performing ATC interference reduction andsingle-user detection, timing information and formed-beam/channelestimates are obtained. Thus, it is possible to reconstruct the MAI andsubtract it from the signal after beam-forming.

Assuming for now that for the k^(th) user, after beam-forming, theformed-beam/channel estimates ({circumflex over (α)}_(k,j),j≠k) areavailable and considering parallel interference cancellation for thek^(th) user, the MAI due to all interferers (j=1 . . . K, j≠k) may bereconstructed by using their corresponding formed beam/channel estimates({circumflex over (α)}_(k,j),j≠k) and bit estimates ({circumflex over(b)}_(j),j≠k). The reconstructed MAI may be subtracted from thebeam-formed signal r_(k). The chip level beam-formed signal can beobtained by applying the weight ŵ_(k) ^(H) in (18) to y_(l) in (6) asfollows:

$\begin{matrix}{\begin{matrix}{r_{k} = {{\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}y_{l}}}\mspace{14mu} \in C^{M \times l}}} \\{= {{\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}{SA}_{l}{bg}_{s}}} + {\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}{PA}_{l}1_{K}g_{p}}} +}} \\{{{\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}V\; A_{l}^{(n)}1_{N}g_{n}}} + {\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}n_{l}}}}} \\{= {{S{\overset{\sim}{A}}_{k}{bg}_{s}} + {P{\overset{\sim}{A}}_{k}1_{K}g_{p}} + {V{\overset{\sim}{A}}_{k}^{(n)}1_{N}g_{n}} + {\overset{\sim}{n}}_{k}}}\end{matrix}{where}} & (30) \\{{\overset{\sim}{A}}_{k} = {\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}A_{l}}}} & (31) \\{{\overset{\sim}{A}}_{k}^{(n)} = {\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}A_{l}^{(n)}}}} & (32) \\{{\overset{\sim}{n}}_{k} = {\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}n_{l}}}} & (33)\end{matrix}$

Note that this beam-formed signal for the k^(th) user is just the ATCcancelled signal, but still has MAI which is contributed from other K−1co-beam/co-frequency users.

As shown in FIG. 12, the interference canceller for the k^(th) user is aspatial combiner 1238 which uses weights ŵ_(k) as in (30) followed bythe correlator 1240 which correlates the received signals r_(k) with thespreading codes s_(k). The interference cancelled bit estimate can beobtained by

{circumflex over (b)} _(k)=Sgn(x _(k) ^((s)))  (34)

where

x _(k) ^((s))=Re(s _(k) ^(H) r _(k))  (35)

3.1. Formed-beam/Channel Estimation

In order to mitigate the MAI, it is first desirable to estimate theformed-beam/channel for each user using the pilot signal. Thebeam-formed signal r_(k) may be applied to a bank of K filters matchedto users' delayed pilot signals p₁ p₂ . . . p_(K) (as follows:

{tilde over (z)} _(k) ^((p)) =P ^(H) r _(k) =R ^((p)) Ã _(k)1_(k) g _(p)+R ^((ps)) Ã _(k) bg _(s) +R ^((pv)) Ã _(k) ^((n))1_(N) g _(n) +P ^(H) ñ_(k) εC ^(K×1)  (36)

If the K-vector {circumflex over (α)}_(k)=[{circumflex over (α)}_(k,1){circumflex over (α)}_(k,2) . . . {circumflex over(α)}_(K,K)]^(T)εC^(K×l) is defined as the formed-beam/channel estimatesfor the k^(th) user, then the {circumflex over (α)}_(k) can be obtainedby normalizing {tilde over (z)}_(k) ^((p)) by the pilot amplitude:

$\begin{matrix}\begin{matrix}{{\hat{a}}_{k} = \frac{{\hat{z}}_{k}^{(p)}}{g_{p}}} \\{= {{{\overset{\sim}{A}}_{k}1_{K}} + {\left( {R^{(p)} - I_{k}} \right){\overset{\sim}{A}}_{k}1_{K}} + {R^{({ps})}{\overset{\sim}{A}}_{k}b\frac{g_{s}}{g_{p}}} +}} \\{{{R^{({pv})}{\overset{\sim}{A}}_{k}^{(n)}1_{N}\frac{g_{n}}{g_{p}}} + {\frac{1}{g_{p}}P^{H}{\overset{\sim}{n}}_{k}}}}\end{matrix} & (37)\end{matrix}$

The formed-beam/channel estimates can be improved by integrating over aperiod of Q pilot symbols so that the residual ATC interference and MAIas well as the noise are low-pass filtered:

$\begin{matrix}\begin{matrix}{{\hat{a}}_{k} = {\frac{1}{{Qg}_{p}}{\sum\limits_{q = 1}^{Q}{\overset{\sim}{z}}_{k,q}^{(p)}}}} \\{= {{{\overset{\sim}{A}}_{k}1_{K}} + {\frac{1}{\sqrt{Q}}\left( {R^{(p)} - I_{k}} \right){\overset{\sim}{A}}_{k}1_{K}} + {\frac{1}{\sqrt{Q}}R^{({ps})}{\overset{\sim}{A}}_{k}b\frac{g_{s}}{g_{p}}} +}} \\{{{\frac{1}{\sqrt{Q}}R^{({pv})}{\overset{\sim}{A}}_{k}^{(n)}1_{N}\frac{g_{n}}{g_{p}}} + {\frac{1}{\sqrt{Q}g_{p}}P^{H}{\overset{\sim}{n}}_{k}}}}\end{matrix} & (38)\end{matrix}$

With the formed-beam/channel estimates for the k^(th) user ({circumflexover (α)}_(k,j),j≠k) and bit estimates ({circumflex over (b)}_(j),j≠k)as well as spreading chip vector (s_(j),j≠k), the MAI term may bereconstructed for interference cancellation.

3.2. Sequential ATC and MAI Interference Cancellation (SAMIC) Detector

A sequential ATC and MAI Interference Cancellation (SAMIC) detector thatmay be configured to reduce interference on a set of multiple accesssignals, according to embodiments of the invention, is based at leastpartially on a realization that MAI cancellation may be more effectivefollowing a reduction of ATC-induced co-channel (and/or non-co-channel)interference and/or other (non ATC-induced) interference that isindependent of the set of multiple access signals. Instead of relying ondetecting a final information associated with a k^(th) multiple accessuser on the interference reduced signal r_(k), the SAMIC detectordetects the final information associated with the k^(th) multiple accessuser based on a further reduction of interference on the interferencereduced signal, obtained by subtracting an estimated of MAI from theinterference reduced signal as illustrated by the formula below:

$\begin{matrix}{{\overset{\sim}{r}}_{k} = {{r_{k} - {\sum\limits_{{j = 1},{j \neq k}}^{K}{{\hat{a}}_{k,j}s_{j}g_{s}{\hat{b}}_{j}}}}\mspace{14mu} \in {C^{M \times l}\mspace{14mu} \left( {k = {1\mspace{14mu} \ldots \mspace{14mu} K}} \right)}}} & (39)\end{matrix}$

where the channel estimates {circumflex over (α)}_(k) are obtained as inequation (38) from the pilot channel following interference reduction(e.g., following stage 1238 in FIG. 12, at stage 1232), and the bitestimates are obtained as in equation (34) following interferencereduction (e.g., following stage 1238 in FIG. 12, at stage 1218).Submitting {circumflex over (α)}_(k) and {circumflex over (b)}_(j),s_(j) (j≠k) to equation (39) yields {tilde over (r)}_(k). TheMAI-reduced {tilde over (r)}_(k) is provided to a correlator that ismatched to the spreading code s_(k). Thus, the maximum-likelihooddetected signal for the k^(th) user is as follows:

$\begin{matrix}{{s_{k}^{H}{\overset{\sim}{r}}_{k}} = {{s_{k}^{H}S{\overset{\sim}{A}}_{k}{bg}_{s}} + {s_{k}^{H}P{\overset{\sim}{A}}_{k}1_{K}g_{p}} + {s_{k}^{H}V{\overset{\sim}{A}}_{k}^{(n)}1_{N}g_{n}} + {\overset{\_}{n}}_{k} - {\sum\limits_{{j = 1},{j \neq k}}^{K}{{\hat{a}}_{k,j}\rho_{k,j}g_{s}{\hat{b}}_{j}}}}} & (40) \\{\mspace{79mu} {where}} & \; \\{\mspace{79mu} {{\rho_{k,j} = {s_{k}^{H}s_{j}}},\left( {k \neq j} \right)}\mspace{79mu}} & (41) \\{\mspace{79mu} {{\overset{\_}{n}}_{k} = {{s_{k}^{H}{\overset{\sim}{n}}_{k}} = {s_{k}^{H}{\sum\limits_{l = 1}^{L}{\left( {\hat{w}}_{k}^{H} \right)_{l}n_{l}}}}}}} & (42)\end{matrix}$

The slicer input provided by the SAMIC detector is given as:

$\begin{matrix}{{{\overset{\sim}{x}}_{k}^{(s)} = {{{Re}\left( {s_{k}^{H}{\overset{\sim}{r}}_{k}} \right)} = {{\gamma_{k}g_{s}} + {ɛ_{k}g_{p}} + {v_{k}g_{n}} + {\overset{\sim}{n}}_{k} - {\delta_{k}g_{s}}}}}{where}{\gamma_{k} = {{Re}\left( {s_{k}^{H}S{\overset{\sim}{A}}_{k}b} \right)}}} & (43) \\{ɛ_{k} = {{Re}\left( {s_{k}^{H}P{\overset{\sim}{A}}_{k}1_{K}} \right)}} & (44) \\{v_{k} = {{Re}\left( {s_{k}^{H}V{\overset{\sim}{A}}_{k}^{(n)}1_{N}} \right)}} & (45) \\{\delta_{k} = {{Re}\left( {\sum\limits_{{j = 1},{j \neq k}}^{K}{{\hat{a}}_{k,j}\rho_{k,j}{\hat{b}}_{j}}} \right)}} & (46) \\{{\overset{\sim}{n}}_{k} = {{Re}\left( {\overset{\_}{n}}_{k} \right)}} & (47)\end{matrix}$

The final decision for the interference cancelled symbol/bit is theoutput of the slicer, namely:

{tilde over ({circumflex over (b)} _(k)=sgn({tilde over (x)} _(k)^((s)))  (48)

Assuming the noise term has the statistics distribution:ñ_(k)˜η(0,∥ŵ_(k)∥²σ²), the final BER for the k^(th) user is given by

$\begin{matrix}{{P_{k}(\sigma)} = {Q\left\lbrack \frac{{{\gamma_{k}g_{s}} + {ɛ_{k}g_{p}} - {\delta_{k}g_{s}}}}{\sqrt{{\sigma^{2}{{\hat{w}}_{k}}^{2}} + {g_{n}{{Var}\left( v_{k} \right)}}}} \right\rbrack}} & (49)\end{matrix}$

4. Simulation Examples

In this section, simulation examples showing the performance of an ATCinterference canceller for single-user detection and the SAMIC detectorfor multi-user detection according to some embodiments of the inventionare presented. Return link adaptive beam-forming with signal inputs fromthe satellite antenna feed elements is considered. The simulation usesthe feed element gain/phase data provided by a satellite manufacturerand a representative ATC footprint over CONUS. The satellite spot-beamsin the forward link are based on fixed beam-forming as provided by thesatellite manufacturer. The forward link fixed spot-beams are only usedhere to illustrate the frequency reuse concept and determine theexclusive zone regions where the co-frequency ATC may be forbidden. FIG.13 illustrates the forward-link spot-beam contours and the location ofATCs, while FIG. 14 illustrates the return-link feed element contoursand the locations of ATCs over CONUS.

4.1 Assumptions and Parameters

The simulation results described herein are based on the cdma2000 1XRTTstandard with Radio Configuration 3 & 4 at rate of 78.6 ksps. The 1XRTTcdma2000 operates at a chip rate of 1.2288 Mcps with channel bandwidthof 1.25 MHz. The spreading gain for the traffic channel is equal to 16(M=16 chips/bit). In particular, for cdma2000, the chip sequence vectorfor pilot channel and traffic signal channel satisfy s_(k)=W₄ ¹⁶•p_(k),where,

W₄ ¹⁶=[+1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1]^(T) is the 16chips of Walsh cover, and (•) denotes the element by element product oftwo same dimension vector or matrix. Other assumptions and parametersinclude:

-   -   1) All ATC interference sources are located at the positions        according to the ATC footprint across CONUS.    -   2) Each ATC source is modeled as an independent point source of        Gaussian noise.    -   3) Each ATC transmits equal power. The total power transmitted        by all ATCs is referred to as “total ATC power that is launched        toward satellite.”    -   4) A total of 175 fixed spot beams for the forward link cover        the continental US, as shown in FIG. 13.    -   5) The frequency reuse cluster size of 3 is considered. The        co-frequency beams are shown in FIG. 13.    -   6) The co-frequency ATC exclusion zone for a beam is defined as        a zone of radius 0.3 (each beam has a radius of 0.2). All ATCs        within an exclusion zone are not allowed to reuse the        frequencies of the satellite beam that is encircled by the        corresponding exclusion zone.    -   7) The return link adaptive beam-forming uses multiple inputs        chosen among 88 feed elements, as shown in FIG. 14.    -   8) The number of receivers (or inputs) is varied from 7 to 35 by        using the feed elements that pick up the most ATCs in each case.    -   9) The maximum signal-to-noise (Eb/No) for the first receiver is        8.4 dB.    -   10) All simulations run 200 frames (20 ms/frame) after        convergence for each point, which is equivalent to a 4 second        length of data.

The traffic channel amplitude g_(s) and the pilot channel amplitudeg_(p) are set according to cdma2000 standard. In the case where only atraffic channel and a pilot channel are transmitted, P_(traffic) isgiven as:

P _(traffic)(dBm)=P _(pilot)(dBm)+0.125×30 dB=P _(pilot)(dBm)+3.75dB  (50)

With the amplitude of the traffic channel g_(s) set to 1.0, theamplitude of the pilot channel g_(p) should be set to 0.65 from equation(50). All involved feed element gains are normalized against the maximumgain of the feed element that picks the most for the desired user.

The ATC interference power is determined by the interference gain g_(n)and variance λ². Since it is assumed that each ATC has equal power, itis possible to set g_(n)=1, (n=1, . . . , N). The relationship betweenλ² and SIR (i.e., the ratio of traffic signal to ATC interference powerlaunched toward the satellite) is given by:

$\begin{matrix}{{SIR} = {10\; {\log_{10}\left( \frac{1}{N\; \lambda^{2}} \right)}}} & (51)\end{matrix}$

The thermal noise variance σ² is determined by

$\frac{E_{b}}{N_{o}}.$

With the processing gain equal to M (M=16), the ratio of

$\frac{E_{b}}{N_{o}}$

is given as:

$\begin{matrix}{\frac{E_{b}}{N_{o}} = {10\; {\log_{10}\left( \frac{M}{\sigma^{2}} \right)}}} & (52)\end{matrix}$

Subject to the above assumptions, simulation results for example casesmay be given.

4.2. Single-User Interference Cancellation Detector

In this section, simulation results based on the single-userinterference cancellation detector according to some embodiments of theinvention are presented. The case where the ATC at each of 50 cities ismodeled as a single point-source will be analyzed first. Then, the casewhere ATCs are modeled as spread point-source clusters will be analyzed.The performance issues will focus on the BER and ΔT/T versus SIR andnumber of feed elements being used for adaptive beam-forming. Inaddition to the assumptions and parameters in 4.1, the simulationresults are based on K=1, μ=0.0001, and Q=1 (i.e., only using 16 chipsintegration for pilot symbol). Though using different μ and/or Q mayyield slightly better or worse performance, the step-size μ is set toμ=0.0001 unless otherwise noted.

Case A—Point-ATC

Assuming the desired mobile user terminal (MT) is located at the centerof the footprint of Feed Element #21 [2.1, 0.05] (i.e., θ=2.1°,φ=0.05°), a total of 16 ATCs are included as co-channel ATCs afterexclusion zone elimination. The feed elements that were used as inputsfor the interference canceller are as below:

-   -   a) 7 Feeds: Feeds #21, 20, 13, 14, 22, 28, 27    -   b) 17 Feeds: in addition to 7 Feeds in a), Feeds #33, 34, 35,        29, 23, 26, 19, 12, 15, 9    -   c) 23 Feeds: in addition to 17 Feeds in b), Feeds #46, 47, 82,        84, 70, 78

FIG. 15 shows the impact of the number of receivers (or feed elements)on BER performance. The performance improves as the number ofreturn-link antenna feed elements (receivers) that are utilizedincreases. However, the case of 23 receivers offers only very slightly(if any) better performance than the case of 17 receivers. This isbecause 17 receivers provide enough degrees of freedom to mitigateco-channel interference from 16 ATCs. As shown in FIG. 15, no errors aredetected when the signal to interference ratio (SIR) is greater than −17dB for the 17 receiver case, and the interference reducer does well inthe region of high interference.

To show the best performance, the BER with 17 receivers is presented inFIG. 16. The step-size μ is set to 0.0002 to improve the performance inthe low interference region. The corresponding ΔT/T vs. SIR plot isshown in FIG. 17. Table 1 gives the values of corresponding ΔT/T.

TABLE 1 ΔT/T vs. SIR SIR (dB) Rxs −47 −42 −37 −32 −27 −22 −17 −12 −7 11.9829e+006 1.068e+005 19426 5902.8 1878.2 594.65 185.35 59.727 17.43717 203.94 73.211 34.186 17.25 6.8503 −2.7462 −19.997 −80.043 −80.043It's noteworthy that ΔT/T is negative until SIR becomes less than −22dB. This appears to be a consequence of desired signal aggregation fromthe plurality of antenna feed elements that are processed.

Return link adaptive beam-forming is accomplished by generating anoptimal beam (i.e., antenna pattern) to null out as many ATC interferesas possible. For the 17 feed elements case, the adaptive beam-formerconverges to a set of weights as shown in Table 2. One complex weight isgenerated for each feed element. These weights form a beam that willcreate a null for each ATC interferer as long as there are sufficientdegrees of freedom. FIGS. 18 and 19 show the beam pattern and contour aswell as the ATC distribution before beam-forming (i.e., using one feedelement—Feed #21). With adaptive beam-forming, the formed-beam patternand contour is shown in FIGS. 20 and 21 respectively. In the contourplots, each contour ring represents a 10 dB of reduction from the verynext inner contour. The effect of interference cancellation is clearlydemonstrated by comparing the plots before and after beam-forming. Atleast one receive antenna feed element of a receive antenna of aspace-based component may be configured to provide two signalscorresponding to two different polarizations of the antenna feedelement. A beam former and/or interference reducer may be configured totake advantage of the two signals to provide polarization diversityprocessing, as will be recognized by those skilled in the art. Thesimulation results presented herein do not include polarizationdiversity processing.

TABLE 2 Beamforming Weights Generated by Interference Canceller FeedWeight ŵ Element # Real part Imaginary part 21 −0.4854 0.025192 200.062945 −0.41016 13 0.26479 0.11987 14 −0.057827 0.089882 22 0.56276−0.12025 28 0.064258 −0.17147 27 −0.15822 −0.10951 33 0.025661 0.07441334 −0.0038039 0.041193 35 −0.035036 −0.072591 29 −0.014305 −0.02951 23−0.22125 0.31089 26 0.21934 0.019156 19 0.078774 0.35891 12 0.0609310.063156 15 −0.021302 −0.054671 9 0.072839 −0.17757

Case B—Spread ATC

In this case, the performance of the interference canceller isinvestigated by expanding each point-source ATC of the previous case toa cluster of 9 ATCs. Each cluster of spread ATCs is uniformlydistributed over a geographic area of 0.05°×0.05° (about 25 Miles×25Miles).

The results for the spread ATC case are compared with the results forthe point-source ATC case by using 23 feed elements in FIG. 22. It canbe seen that the spread ATC does not have much impact on performancewhen SIR is greater than −22 dB. However, when interference is gettingstronger than that, the ATC spread effect becomes evident. Thecorresponding ΔT/T vs. SIR is given in Table 3.

TABLE 3 ΔT/T vs. SIR SIR (dB) Rxs −37 −32 −27 −22 −17 −12 −7  1 194265902.8 1878.2 594.65 185.35 59.727 17.437 23 30.557 20.319 16.035 4.5041−21.782 −80.043 −80.043 (Point ATC) 23 224.06 77.623 29.784 5.8623−17.988 −80.043 −80.043 (spread ATC)Note that the above results were obtained by using p=0.0001. The ΔT/Tvalue reaches about 6% when SIR is approximately −22 dB for the spreadATC case. If μ is doubled to 0.0002, the results improve as in the caseshown in Table 2.

Case C—Moving MT Location

Here the mobile terminal location is moved from the maximum feed elementgain location [2.1, 0.05] in Case A to [2.2, 0.15] in this case.Assuming the MT still transmits the same power, the received E_(b)/N_(o)from Feed #21 is reduced by 0.8 dB due to the MT now being off the peakof Feed #21. Hence in this case E_(b)/N_(o) is 7.6 dB. The feed elementsused for beam-forming remain the same as in Case A.

FIG. 23 shows BER performance versus SIR as the number of feed elementsvaries from 1 to 23. Again when the number of feed elements is greaterthan 17, the performance very much converges. The spread ATC effect isshown in FIG. 24 for the 23 feed elements case. Table 4 lists the ΔT/Tvs. SIR for both point-ATC and spread ATC with 23 feed elements.

TABLE 4 ΔT/T vs. SIR SIR (dB) Rxs −37 −32 −27 −22 −17 −12 −7  1 197255939.8 1885.5 595.58 187.15 60.028 19.196 23 20.954 12.18 7.1505 −4.6035−21.398 −39.007 −83.4 (Point ATC) 23 195.34 63.775 21.837 −1.3111−22.366 −36.728 −83.4 (spread ATC)

4.3 SAMIC Multi-User Detector

In this section, simulation results for the use of a SAMIC multi-userdetector in a multi-user environment under ATC interference arepresented. It is assumed that the co-beam multiple users are randomlyuniform-distributed inside Beam #122. The ATC interference footprint andsatellite feed elements remain the same as in the previous single usercase. In addition to considering the cdma2000 reverse traffic channelwith spreading gain of 16 at data rate of 78.6 kbps with the assumptionsand parameters of section 4.1, simulation results are also included forspreading gains of 32 and 64 (M=32 chips/bit and 64 chips/bit) at datarates of 38.4 kbps and 19.2 kbps, respectively. For the case ofspreading gain of 32, it is assumed that the chip sequence vector forpilot channel and traffic signal channel satisfy S_(k)=W₈ ³²•p_(k),where W₈ ³² is the 32 chips of Walsh cover, and (•) denotes the elementby element product of two same dimension vector or matrix. Similarly,for the case of spreading gain of 64, the chip sequence vector for pilotchannel and traffic signal channel satisfy S_(k)=W₁₆ ⁶⁴•p_(k), where W₁₆⁶⁴ denotes the 64 chips of Walsh cover. All K users inside Beam #122 areassumed to have equal EIRP. It is further assumed that each user has thesame

$\frac{E_{b}}{N_{o}} = {8.4\mspace{14mu} {{dB}.}}$

FIG. 25 shows the uniformly distributed random locations of 50 usersinside beam #122 (overlapping with Feed #21) along with ATC footprintand Feed #21 gain pattern contour. A total of 16 co-frequency ATCs areincluded after exclusion zone elimination. The feed elements that wereused as inputs for the interference canceller are as below:

a) For the one receiver, i.e., L=1 case: Feeds #21.

b) For the 17 receivers, i.e., L=17 case: Feeds: #21, 20, 13, 14, 22,28, 27, 33, 34, 35, 29, 23, 26, 19, 12, 15, and 9.

Case A—Spreading Gain M=16 (cdma2000 RC 3 & 4)

This is the case defined by cdma2000 Radio Configuration 3 & 4 for thedata rate of 78.6 kbps. A situation of 5 co-beam MT users (K=5) thathave the same EIRP is addressed initially. FIG. 26 shows an average ofBERs for all five users versus SIR that is defined as satellite signalto ATC power ratio that is launched toward a satellite. In FIG. 26 thesimulation results from the single user detector (SUD) and SAMIC andSAMIC2 detectors with one receiver and 17 receivers are given. TheSAMIC2 detector is a two-stage SAMIC detector where the second stageSAMIC uses the bit estimate from the first stage SAMIC as its bitestimate input. Unlike the SAMIC detector whose bit estimate input isfrom the output decision of the ATC interference canceller, the secondstage SAMIC uses the bit estimates from the first stage SAMIC to furtherimprove the multi-user detection performance. In the one receiver case,the SAMIC detector only shows advantage over SUD when the ATCinterference decreases to certain level. However, in the case of 17receivers, the benefit of the SAMIC is significant compared with the SUDdetector.

The performance of SAMIC2 detector is slightly improved over the SAMICdetector. To optimize the performance in this scenario, we set μ=0.0002,and Q=1 (i.e., using 16 chips integration for pilot symbol) for the LMSalgorithm, and Q=96 (i.e., using 1536 chips, or 1 PCG integration forchannel estimation) for the SAMIC detector. FIG. 27 gives the average ofBERs versus the number of active users when SIR=−12 dB. For the onereceiver case (without ATC interference cancellation), the SAMIC andSAMIC2 detectors would be worse than SUD when K is greater that 25 thisis because the combination of ATC and MAI interference would make theSAMIC detector produce more errors when there is no ATC interferencecancellation. With ATC interference cancellation (i.e., the 17 receiverscase), the SAMIC detector demonstrates superior performance over SUD.The best performing SAMIC2 detector exceeds 1% BER when K is greaterthan 20. To improve the capacity, it may be desirable to increase thespreading gain.

Case B—Spreading Gain M=32

In this case, the spreading gain is increased to 32, which effectivelyleads to the traffic data rate of 38.4 kbps. The same simulationassumptions and parameters as in Case A are used, except that the chipintegration length for LMS is optimized to 32 chips and the chipintegration length for channel estimation is optimized to 3072 chips (2PCGs) with proportion to the spreading gain because

$\frac{E_{b}}{N_{o}}$

is fixed to 8.4 dB for each user. Considering the first 10 activeco-beam equal power users from the 50 user profile, the average of BERsfor the 10 users versus SIR is shown in FIG. 28. It appears that theSAMIC and SAMIC2 detectors provide better performance than SUD acrossthe SIR range for the 17 receivers case. The average of BERs for theSAMIC2 detector ranges from 10⁻⁴ for SIR=0 dB to 6×10⁻³ for SIR=−40 dB.FIG. 29 shows the average of BERs versus the number of active co-beamusers when SIR=−10 dB. Comparing with the M=16 case, it is clear thatincreasing the spreading gain makes the SAMIC/SAMIC2 detector moreeffective in both one receiver and 17 receivers. The average BERs forthe SAMIC2 detector is still under 1% when K=40. To look into the BERperformance among all active users, the BER spread of the SAMIC2detector is provided with the maximum and minimum on top of the averagedBER for L=17 case in FIG. 30. The significant performance improvementprovided by SAMIC2 over SUD is demonstrated.

Case C—Spreading Gain M=64

To further evaluate performance versus spreading gain, the spreadinggain may be increased to 64 while still having a reasonable 19.2 kbps oftraffic data rate. Again the simulation assumptions and parameters arethe same as in Case B. To optimize the performance, the chip integrationlength for LMS can be increased to 64 chips and chip integration lengthfor channel estimation to 6144 chips (4 PCGs, which appears onlyslightly better than 2 PCGs). FIG. 31 shows the average of BERs for thefirst 10 users from the 50 users profile versus SIR. The SAMIC andSAMIC2 detectors outperform the SUD detector significantly. Since only10 users are considered, the benefit of the SAMIC2 detector over theSAMIC detector does not appear for the high processing case. However,the advantage of the SAMIC2 over the SAMIC detector can be more evidentas the number of users increases. Both the SAMIC and SAMIC2 detectorsperform better than SUD even with only one receiver because of highprocessing gain. The average BER versus number of users K is given forSIR=−10 dB in FIG. 32.

It can be seen that the more active users the more the apparentadvantage of SAMIC2 over SAMIC for the number of users range that wasconsidered. The SAMIC2 detector can keep average BER bellow 10⁻³ as thenumber of users approaches 50. FIG. 33 gives average BER along with themaximum and minimum of BERs among all involved users for the L=17 case.Again the SAMIC2 detector outperforms the SUD significantly across therange. In the best scenario, the SAMIC detector provides 6.5×10⁻⁵ BERfor K=45 and 2.3×10⁻⁴ BER for K=50.

Return link adaptive beamforming has been analyzed in conjunction withmulti-user detection for satellite based CDMA system. A set of equationshas been presented to illustrate an algorithm to cancel both ATCinterference and MAI interference under intra-beam multi-userenvironment. Several simulation examples have shown the performance ofthe ATC interference canceller for single-user and the SAMIC detectorfor multi-user with a set of satellite feed element inputs and the ATCfootprint over CONUS.

The LMS interference algorithm is based on the use of a desired user'spilot signal to minimize the impact of spatial ATC interferers. It hasbeen shown that the LMS algorithm can effectively mitigate ATCinterference for both point-source ATC and spread ATCs. The interferencecanceller may use about 17 feed element inputs and proper step-size andintegration length for LMS. Using more than 17 feed elements may onlyprovide slight improvement and very much converges for performance.However, the spatially operated LMS does not appear to be able to cancelthe multiple access interference. The SAMIC detector has been presentedto provide sequential ATC interference cancellation and MAIcancellation. In the intra-beam multi-user situation, the SAMIC detectortakes advantage of known ATC cancelled bit estimates and spreading codesequence/timing as well as channel estimates to efficiently enable ATCinterference cancellation and MAI mitigation sequentially. Inconjunction with the LMS algorithm, the SAMIC detector can significantlyboost system capacity compared with the SUD detector, depending onspreading gain. By using a second stage SAMIC, the SAMIC2 detector canimprove the performance even further. The channel estimation is obtainedby using pilot matching filter on beam-formed chip level signal andintegration over an interval of time. The integration length for channelestimation appears to be a number of PCGs in proportion with spreadinggain when E_(b)/N_(o) is fixed. For the M=16 case, the SAMIC2 detectormay tolerate about 15 users for SIR=−12 dB. By doubling the spreadinggain to 32, the SAMIC2 detector can increase capacity to 40 users forSIR=−10 dB. Finally for the case of a spreading gain of 64 with SIR=−10dB, the SAMIC2 detector has the average BER of 10⁻³ for 50 users.

It will be understood that any air interface protocol may be used by aspace-based component to provide space-based communications. Similarly,it will be understood that any air interface protocol may be used by anancillary terrestrial network to provide terrestrial communicationswhile using/reusing terrestrially at least some of the frequenciesauthorized for use by the space-based component. In some embodiments,the air interface protocol for the space-based component may beGSM-based while the air interface protocol for the ancillary terrestrialnetwork may be CDMA-based.

In the drawings and/or the specification, there have been disclosedembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

1. A communications method comprising: receiving at a space-basedcomponent over a space-based component frequency band a plurality ofsignals including interference that is dependent on transmissions of aplurality of terminals communicating with the space-based component overa geographic area and interference that is independent of thetransmissions of the plurality of terminals communicating with thespace-based component over the geographic area; and reducinginterference of the plurality of signals sequentially by first reducingthe interference that is independent of the transmissions of theplurality of terminals communicating with the space-based component overthe geographic area and then reducing the interference that is dependenton the transmissions of the plurality of terminals communicating withthe space-based component over the geographic area.
 2. The method ofclaim 1 wherein the interference that is independent of thetransmissions of the plurality of terminals communicating with thespace-based component over the geographic area comprises interferencethat is dependent on transmissions by an ancillary terrestrial networkand/or by one or more terminals communicating with the ancillaryterrestrial network, and wherein the ancillary terrestrial networkand/or the one or more terminals communicating therewith use at leastsome frequencies of the space-based component.
 3. The method of claim 1,wherein receiving at the space-based component comprises receiving usinga plurality of antenna feed elements.
 4. The method of claim 3, whereinreducing interference of the plurality of signals sequentially comprisesprocessing pilot signals and determining weights for the plurality ofantenna feed elements based on the processing of the pilot signals. 5.The method of claim 4, further comprising generating at least one pilotsignal error based on the processing.
 6. The method of claim 5, whereinthe weights for the antenna feed elements are selected to reduce a meansquared measure of the at least one pilot signal error.
 7. The method ofclaim 4, further comprising applying the weights to signals received bythe antenna feed elements to obtain interference reduced signals.
 8. Themethod of claim 7, wherein reducing the interference that is dependenton the transmissions of the plurality of terminals communicating withthe space-based component over the geographic area comprises determininga set of channel estimates based on the interference reduced signals,generating a set of bit estimates from the interference reduced signals,and performing interference reduction on the interference reducedsignals using the set of channel estimates and the bit estimates.
 9. Themethod of claim 8, wherein performing interference reduction on theinterference reduced signals using the channel estimates and the bitestimates comprises generating second interference reduced signals, andwherein the method further comprises determining a set of second channelestimates based on the second interference reduced signals, generating aset of second bit estimates from the second interference reducedsignals, and performing interference reduction on the secondinterference reduced signals using the second channel estimates and thesecond bit estimates.
 10. The method of claim 1, wherein receiving atthe space-based component comprises receiving at the space-basedcomponent using at least two antenna patterns that differ in at least apolarization orientation.
 11. The method of claim 8, wherein generatinga set of bit estimates from the interference reduced signals comprisescorrelating an interference reduced signal with a spreading code. 12.The method of claim 1, wherein reducing the interference that isdependent on transmissions of a plurality of terminals communicatingwith the space-based component over a geographic area comprisesgenerating bit estimates, and the method further comprises performinginterference reduction using the generated bit estimates.
 13. The methodof claim 1, further comprising transmitting at least some of theplurality of signals to at least one satellite gateway, wherein reducinginterference of the plurality of signals is performed at the at leastone satellite gateway.
 14. The method of claim 13, wherein the at leastone satellite gateway is terrestrially based.
 15. The method of claim 1,wherein reducing interference of the plurality of signals is performedat least partially at the space-based component and at least partiallyat a satellite gateway that is terrestrially based.
 16. A systemcomprising: a plurality of terminals that are configured to transmit aplurality of multiple access signals over a space-based componentfrequency band in a space-based component footprint; a space-basedcomponent that is configured to receive the plurality of multiple accesssignals over the space-based component frequency band, the space-basedcomponent also receiving interference along with the plurality ofmultiple access signals in the space-based component frequency band, theinterference comprising a component that is dependent on the pluralityof multiple access signals and a component that is independent of theplurality of multiple access signals; and an interference reducingelement that is responsive to the space-based component, and that isconfigured to sequentially perform interference reduction of theinterference that is independent of the plurality of multiple accesssignals and the interference that is dependent on the plurality ofmultiple access signals.
 17. The system of claim 16, further comprising:an ancillary terrestrial network including a plurality of transmittersconfigured to transmit a plurality of waveforms over at least some ofthe space-based component frequency band, and wherein the interferencethat is independent of the plurality of multiple access signalscomprises a component that is dependent on the plurality of waveforms.18. The system of claim 16, wherein the space-based component includesan antenna having a plurality of antenna feed elements, and thespace-based component is configured to receive the plurality of multipleaccess signals using the antenna having the plurality of antenna feedelements.
 19. The system of claim 18, wherein the interference reducingelement is further configured to perform interference reduction on aplurality of multiple access signals by processing pilot signalstransmitted by a plurality of terminals and determining a set of weightsfor the antenna feed elements based on the processing of the pilotsignals.
 20. The system of claim 19, wherein the processing generates atleast one pilot signal error.
 21. The system of claim 20, wherein theinterference reducing element is further configured to select a set ofweights for the antenna feed elements to reduce a mean squared measureof the at least one pilot signal error.
 22. The system of claim 21,wherein the interference reducing element is further configured to applythe set of weights to signals received by the plurality of antenna feedelements to obtain interference reduced signals.
 23. The system of claim22, wherein the interference reducing element is further configured todetermine a set of channel estimates based on the interference reducedsignals, generate a set of bit estimates from the interference reducedsignals, and perform interference reduction on the interference reducedsignals using the set of channel estimates and the set of bit estimatesto thereby generate second interference reduced signals.
 24. The systemof claim 23, wherein the interference reducing element is furtherconfigured to determine a set of second channel estimates based on thesecond interference reduced signals, to generate a set of second bitestimates from the second interference reduced signals, and to performinterference reduction on the second interference reduced signals usingthe set of second channel estimates and the set of second bit estimates.25. The system of claim 16, wherein the space-based component receivesthe plurality of multiple access signals using at least two antennapatterns that differ in at least a polarization orientation.
 26. Thesystem of claim 22, wherein the interference reducing element is furtherconfigured to generate a plurality of interference reduced bit estimatesfrom the interference reduced signals, and to perform interferencereduction using the plurality of interference reduced bit estimates. 27.The system of claim 16, wherein the space-based component is furtherconfigured to re-transmit the multiple access signals to a satellitegateway, and wherein the interference reducing element is located at thesatellite gateway.
 28. The system of claim 27, wherein the satellitegateway is terrestrially-based.
 29. An interference reducer for aspace-based component that is configured to receive waveforms fromradioterminals including interference that is dependent on the waveformsfrom the radioterminals and interference that is independent of thewaveforms from the radioterminals, wherein the interference reducer isconfigured to reduce the interference that is independent of thewaveforms from the radioterminals and to reduce the interference that isdependent on the waveforms from the radioterminals; and wherein theinterference reducer is configured to reduce the interference that isdependent on the waveforms from the radioterminals repeatedly until apredetermined criterion is met.
 30. The method of claim 1, furthercomprising repeatedly reducing the interference that is dependent on thetransmissions of the plurality of terminals communicating with thespace-based component over the geographic area until a predeterminedcriterion is met.
 31. The method of claim 30, wherein the predeterminedcriterion comprises a bit error rate.